# EDGAR Filing Document

**Accession Number:** 0001390478
**File Stem:** 0001104659-25-098711
**Filing Date:** 2025-10
**Character Count:** 92830
**Document Hash:** 2baf9c502121d4eda2ae1a42f7e2dcb9
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001104659-25-098711.hdr.sgml**: 20251010

**ACCESSION NUMBER**: 0001104659-25-098711

**CONFORMED SUBMISSION TYPE**: S-3

**PUBLIC DOCUMENT COUNT**: 16

**FILED AS OF DATE**: 20251010

**DATE AS OF CHANGE**: 20251010

**FILER**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** SELLAS Life Sciences Group, Inc.
- **CENTRAL INDEX KEY:** 0001390478
- **STANDARD INDUSTRIAL CLASSIFICATION:** PHARMACEUTICAL PREPARATIONS [2834]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 000000000
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** S-3
- **SEC ACT:** 1933 Act
- **SEC FILE NUMBER:** 333-290829
- **FILM NUMBER:** 251388161

**BUSINESS ADDRESS:**
- **STREET 1:** 7 TIMES SQUARE
- **STREET 2:** SUITE 2503
- **CITY:** NEW YORK
- **STATE:** NY
- **ZIP:** 10036
- **BUSINESS PHONE:** 6462005278

**MAIL ADDRESS:**
- **STREET 1:** 7 TIMES SQUARE
- **STREET 2:** SUITE 2503
- **CITY:** NEW YORK
- **STATE:** NY
- **ZIP:** 10036

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Galena Biopharma, Inc.
- **DATE OF NAME CHANGE:** 20110926

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** RXi Pharmaceuticals Corp
- **DATE OF NAME CHANGE:** 20110926

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** GALENA BIOPHARMA, INC.
- **DATE OF NAME CHANGE:** 20110922

**As filed with the Securities and Exchange Commission on October 10, 2025**

**Registration No. 333-**

**UNITED STATES**

**SECURITIES AND EXCHANGE COMMISSION**

**WASHINGTON, D.C. 20549**

**FORM S-3**

**REGISTRATION STATEMENT**

***UNDER***

***THE SECURITIES ACT OF 1933***

**SELLAS Life Sciences Group, Inc.**

**(Exact name of registrant as specified in its charter)**

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| | |
|:---|:---|
| &nbsp;&nbsp; **Delaware**<br> **(State or other jurisdiction of<br> incorporation or organization)** | &nbsp;&nbsp; **20-8099512**<br> **(I.R.S. Employer Identification**<br> **Number)** |

---

**7 Times Square, Suite 2503**

**New York, NY 10036**

**(646) 200-5278**

(Address, including zip code, and telephone number, including area code, of registrant's principal executive offices)

**Angelos M. Stergiou, M.D., ScD h.c.**

**President and Chief Executive Officer**

**7 Times Square, Suite 2503**

**New York, NY 10036**

**(646) 200-5278**

(Name, address, including zip code, and telephone number, including area code, of agent for service)

***Copies to:***

**Daniel A. Bagliebter, Esq.**

**Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.**

**919 Third Avenue**

**New York, New York 10022**

**(212) 935-3000**

**Approximate date of commencement of proposed sale to the public**: From time to time after the effective date of this registration statement.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box: ◻

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box: ⌧

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ◻

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ◻

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box. ◻

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ◻

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of "large accelerated filer," "accelerated filer," "smaller reporting company" and "emerging growth company" in Rule 12b-2 of the Exchange Act.

Large accelerated filer ◻ Accelerated filer ◻ <br> Non-accelerated filer ⌧ Smaller reporting company ⌧ <br> Emerging growth company ◻

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ◻

**The Registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment that specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until this registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.**

**The information in this prospectus is not complete and may be changed. A registration statement relating to these securities has been filed with the Securities and Exchange Commission. We may not sell these securities or accept an offer to buy these securities until the registration statement is effective. This prospectus is not an offer to sell these securities and it is not soliciting offers to buy these securities in any jurisdiction where the offer or sale is not permitted.**

**PRELIMINARY PROSPECTUS - SUBJECT TO COMPLETION, DATED OCTOBER 10, 2025**

![](tm2528102d1_s3img001.jpg)

**SELLAS LIFE SCIENCES GROUP, INC.**

**19,685,040 Shares of Common Stock**

This prospectus covers the offer and resale by the selling stockholders identified in this prospectus of up to an aggregate of 19,685,040 shares of our common stock, $0.0001 par value per share (the "Common Stock"), consisting of 19,685,040 shares of Common Stock issuable upon the exercise of Warrants to purchase shares of Common Stock (collectively, the "Warrants"). We are registering the shares issuable upon exercise of the Warrants on behalf of the selling stockholders, to be offered and sold by them from time to time.

We are not selling any shares of Common Stock under this prospectus and will not receive any proceeds from the sale by the selling stockholders of such shares. We are paying the cost of registering the shares of Common Stock covered by this prospectus as well as various related expenses. The selling stockholders are responsible for all selling commissions, transfer taxes and other costs related to the offer and sale of their shares.

Sales of the shares by the selling stockholders may occur at fixed prices, at market prices prevailing at the time of sale, at prices related to prevailing market prices, at negotiated prices and/or at varying prices determined at the time of sale. The selling stockholders may sell shares directly or to or through underwriters, broker-dealers or agents, who may receive compensation in the form of discounts, concessions or commissions from the selling stockholders, the purchasers of the shares, or both. The selling stockholders may sell any, all or none of the securities offered by this prospectus, and we do not know when or in what amount the selling stockholders may sell their shares of Common Stock hereunder following the effective date of the registration statement of which this prospectus forms a part. We provide more information about how the selling stockholders may sell or otherwise dispose of their shares of Common Stock in the section titled "<u>Plan of Distribution</u>" on page 9.

Our Common Stock is listed on The Nasdaq Capital Market under the symbol "SLS." On October 9, 2025, the last reported sale price of our Common Stock was $1.8950 per share.

**Investing in our securities involves a high degree of risk. You should carefully consider the risks described under "<u>Risk Factors</u>" beginning on page 6 of this prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus.**

**Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.**

**The date of this prospectus is , 2025**

**TABLE OF CONTENTS**

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| | |
|:---|:---|
|  | Page |
| [ABOUT THIS PROSPECTUS](#a_002) | [ii](#a_002) |
| [SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS](#a_003) | [iii](#a_003) |
| [PROSPECTUS SUMMARY](#a_004) | [1](#a_004) |
| [THE OFFERING](#a_005) | [5](#a_005) |
| [RISK FACTORS](#a_006) | [6](#a_006) |
| [USE OF PROCEEDS](#a_007) | [7](#a_007) |
| [SELLING STOCKHOLDERS](#a_008) | [7](#a_008) |
| [PLAN OF DISTRIBUTION](#a_009) | [9](#a_009) |
| [LEGAL MATTERS](#a_010) | [11](#a_010) |
| [EXPERTS](#a_011) | [11](#a_011) |
| [WHERE YOU CAN FIND MORE INFORMATION](#a_012) | [11](#a_012) |
| [INCORPORATION OF CERTAIN INFORMATION BY REFERENCE](#a_013) | [11](#a_013) |

---

i

**ABOUT THIS PROSPECTUS**

This prospectus is part of a registration statement on Form S-3 that we filed with the Securities and Exchange Commission (the "SEC") pursuant to which the selling stockholders may sell, from time to time in one or more offerings, the Common Stock described in this prospectus.

We have not authorized anyone to provide you with information other than the information that we have provided or incorporated by reference in this prospectus and your reliance on any unauthorized information or representation is at your own risk. This prospectus may be used only in jurisdictions where offers and sales of these securities are permitted. You should assume that the information appearing in this prospectus is accurate only as of the date of this prospectus and that any information we have incorporated by reference is accurate only as of the date of the document incorporated by reference, regardless of the time of delivery of this prospectus, or any sale of our Common Stock. Our business, financial condition and results of operations may have changed since those dates.

Unless otherwise stated, all references in this prospectus to "we," "us," "our," "SELLAS," the "Company" and similar designations refer to SELLAS Life Sciences Group, Inc. This prospectus contains references to trademarks belonging to other entities. Solely for convenience, trademarks and trade names referred to in this prospectus, including logos, artwork and other visual displays, may appear without the® or™ symbols, but such references are not intended to indicate, in any way, that the applicable licensor will not assert, to the fullest extent under applicable law, its rights to these trademarks and trade names. We do not intend our use or display of other companies' trade names or trademarks to imply a relationship with, or endorsement or sponsorship of us by, any other companies.

A prospectus supplement may add to, update or change the information contained in this prospectus. You should read both this prospectus and any applicable prospectus supplement together with additional information described below under the heading "Where You Can Find Additional Information."

ii

**SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS**

This prospectus and the documents incorporated by reference contain forward-looking statements about us and our industry that involve substantial risks and uncertainties. All statements other than statements of historical facts contained in this prospectus and the documented incorporated by reference, including statements regarding our future financial condition, business strategy and plans, and objectives of management for future operations, are forward-looking statements. In some cases, you can identify forward-looking statements by terminology such as "aim," "anticipate," "assume," "believe," "contemplate," "continue," "could," "design," "due," "estimate," "expect," "goal," "intend," "may," "objective," "plan," "predict," "positioned," "potential," "seek," "should," "target," "will," "would" and other similar expressions that are predictions of or indicate future events and future trends, or the negative of these terms or other comparable terminology.

We have based these forward-looking statements largely on our current expectations and projections about future events and financial trends that we believe may affect our financial condition, results of operations, business strategy and financial needs. These forward-looking statements are subject to a number of known and unknown risks, uncertainties and assumptions, including risks described in the section titled "Risk Factors" contained in our most recent Annual Report on Form 10-K and Quarterly Reports on Form 10-Q and incorporated by reference in this prospectus, as the same may be amended, supplemented or superseded by the risks and uncertainties described under similar headings in the other documents that are filed after the date hereof and incorporated by reference into this prospectus, regarding, among other things:

&nbsp;&nbsp;&nbsp;&nbsp;· our future financial and business performance;

&nbsp;&nbsp;&nbsp;&nbsp;· strategic plans for our business and product candidates;

&nbsp;&nbsp;&nbsp;&nbsp;· our ability to develop or commercialize products;

&nbsp;&nbsp;&nbsp;&nbsp;· the expected results and timing of clinical trials and nonclinical studies;

&nbsp;&nbsp;&nbsp;&nbsp;· our ability to comply with the terms of our license agreements;

&nbsp;&nbsp;&nbsp;&nbsp;· developments and projections relating to our competitors and industry;

&nbsp;&nbsp;&nbsp;&nbsp;· our expectations regarding our ability to obtain, develop and maintain intellectual property protection and not infringe on the rights of others;

&nbsp;&nbsp;&nbsp;&nbsp;· our ability to retain and attract highly-skilled executive officers and employees;

&nbsp;&nbsp;&nbsp;&nbsp;· our future capital requirements and the timing of those requirements and sources and uses of cash;

&nbsp;&nbsp;&nbsp;&nbsp;· our ability to obtain funding for our operations;

&nbsp;&nbsp;&nbsp;&nbsp;· changes in the assumptions underlying our expectations regarding our future business or business model;

&nbsp;&nbsp;&nbsp;&nbsp;· risks associated with preclinical or clinical development and trials;

&nbsp;&nbsp;&nbsp;&nbsp;· our ability to develop, manufacture and commercialize product candidates;

&nbsp;&nbsp;&nbsp;&nbsp;· general economic, financial, legal, political and business conditions and changes in domestic and foreign markets;

&nbsp;&nbsp;&nbsp;&nbsp;· changes in applicable laws or regulations;

&nbsp;&nbsp;&nbsp;&nbsp;· the impact of natural disasters, including climate change, and the impact of health epidemics, on our business;

&nbsp;&nbsp;&nbsp;&nbsp;· the size and growth potential of the markets for our products, and our ability to serve those markets;

iii

&nbsp;&nbsp;&nbsp;&nbsp;· market acceptance of our planned products;

&nbsp;&nbsp;&nbsp;&nbsp;· our ability to raise capital; and

&nbsp;&nbsp;&nbsp;&nbsp;· the possibility that we may be adversely affected by other economic, business, and/or competitive factors.

These risks are not exhaustive. Other sections of this prospectus, or the documents incorporated herein by reference may include additional factors that could harm our business and financial performance. Moreover, we operate in a very competitive and rapidly changing environment. New risk factors emerge from time to time, and it is not possible for our management to predict all risk factors, nor can we assess the impact of all factors on our business or the extent to which any factor, or combination of factors, may cause actual results to differ materially from those contained in, or implied by, any forward-looking statements.

You should not rely upon forward-looking statements as predictions of future events. We cannot assure you that the events and circumstances reflected in the forward-looking statements will be achieved or occur. Although we believe that the expectations reflected in the forward-looking statements are reasonable, we cannot guarantee future results, levels of activity, performance or achievements. Except as required by law, we undertake no obligation to update publicly any forward-looking statements for any reason after the date of this prospectus or to conform these statements to actual results or to changes in our expectations.

You should carefully read this prospectus, and the applicable prospectus supplement, together with the information incorporated herein by reference as described under the heading "Incorporation by Reference," as well as the documents filed as exhibits to the registration statement of which this prospectus is a part with the understanding that our actual future results, levels of activity, performance and achievements may be materially different from what we expect. We qualify all of our forward-looking statements by these cautionary statements.

Except as required by law, we assume no obligation to update these forward-looking statements publicly, or to update the reasons actual results could differ materially from those anticipated in these forward-looking statements, even if new information becomes available in the future.

iv

**PROSPECTUS SUMMARY**

*This summary highlights information contained elsewhere in this prospectus or incorporated by reference in this prospectus. This summary provides an overview of selected information and does not contain all of the information you should consider before investing in our securities. You should read this entire prospectus and the applicable prospectus supplement carefully, especially the sections titled "Risk Factors" and our consolidated financial statements and related notes included elsewhere in this prospectus, the applicable prospectus supplement and the documents incorporated by reference therein before making an investment decision. Except as otherwise indicated or unless the context otherwise requires, references to "company," "we," "us," "our" or "SELLAS," refer to SELLAS Life Sciences Group, Inc. and its consolidated subsidiaries.*

 

***Overview***

We are a late-stage clinical biopharmaceutical company focused on the development of novel therapeutics for a broad range of cancer indications. Our product candidates currently include galinpepimut-S, or GPS, a peptide immunotherapy directed against the Wilms tumor 1, or WT1, antigen, and SLS009 (tambiciclib), a highly selective small molecule cyclin-dependent kinase 9, or CDK9, inhibitor.

*Galinpepimut-S, or GPS: Highly Novel and Engineered Immunotherapy Targeting the WT1 Antigen*

Our lead product candidate, GPS, is a cancer immunotherapeutic agent licensed from Memorial Sloan Kettering Cancer Center, or MSK, that targets the WT1 protein, which is present in 20 or more cancer types. Based on its mechanism of action as a directly immunizing agent, GPS has potential as a monotherapy or in combination with other immunotherapeutic agents to address a broad spectrum of hematologic, or blood, cancers, and solid tumor indications.

We have an ongoing open label randomized Phase 3 clinical trial, the REGAL study, for GPS monotherapy in patients with acute myeloid leukemia, or AML, in the maintenance setting after achievement of second complete remission, or CR2, following successful completion of second-line antileukemic therapy. Patients are randomized to receive either GPS or best available treatment, or BAT. We expect this study will be used as the basis for submission of a Biologics License Application, or BLA, subject to a statistically significant and clinically meaningful trial outcome and agreement with the U.S. Food and Drug Administration, or the FDA. The primary endpoint of the REGAL study is overall survival, or OS. In March 2024, we announced the completion of enrollment of 126 patients at clinical sites in the U.S., Europe, and Asia with a planned interim safety, efficacy and futility analysis after 60 events (deaths). Study sites in the U.S. and Europe account for approximately, 75% of patients enrolled, with the U.S.-based sites representing the highest enrolling country. In December 2024, we announced that the pre-specified threshold of 60 events (deaths) per the protocol had been reached, triggering the interim analysis to be conducted by the Independent Data Monitoring Committee, or IDMC. In January 2025, we announced that the IDMC had completed pre-specified interim analysis of the REGAL study and had recommended that the study continue without modifications. In August 2025, we announced the completion of a periodic review by the IDMC and positive recommendation to continue the REGAL study without modification. The final analysis will be conducted once 80 events (deaths) are reached. We anticipate that 80 events will be reached this year. Because the final analysis is event driven, it is difficult to predict with any certainty and it may occur at a different time than currently expected.

In December 2020, we entered into an exclusive license agreement, or the 3D Medicines Agreement, with 3D Medicines Inc., or 3D Medicines, a China-based biopharmaceutical company developing next-generation immuno-oncology drugs, for the development and commercialization of GPS, as well as the Company's next generation heptavalent immunotherapeutic GPS+, which is at preclinical stage, across all therapeutic and diagnostic uses in mainland China, Hong Kong, Macau and Taiwan, which we refer to as Greater China. We have retained sole rights to GPS and GPS+ outside of Greater China. In November 2022, we announced that we had agreed with 3D Medicines for 3D Medicines to participate in the REGAL study through the inclusion of approximately 20 patients from mainland China. In December 2022, we entered into a Side Letter Agreement with 3D Medicines, or Side Letter, which together with the 3D Medicines Agreement, details the terms and conditions of 3D Medicines' participation in the REGAL study. Although the REGAL study has completed enrollment as announced in March 2024, in accordance with the predetermined statistical analysis plan, 3D Medicines may still enroll patients in mainland China. The timing of such participation and patient enrollment by 3D Medicines, if at all, cannot be predicted with certainty. As of March 15, 2025, we have received an aggregate of $10.5 million in upfront and milestone payments under the 3D Medicines Agreement and a total of $191.5 million in potential future development, regulatory and sales milestones, not including future royalties, remains under the license agreement, which milestones are variable in nature and not under our control. In December 2023, we announced that we had commenced a binding arbitration proceeding against 3D Medicines to resolve a dispute regarding, among other things, the trigger and payment of relevant milestone payments due to us under the 3D Medicines Agreement.

GPS was granted Orphan Drug Designations, or ODD, from the FDA, as well as orphan medicines designations from the European Medicines Agency, or EMA, in AML, malignant pleural mesothelioma, or MPM, and multiple myeloma, or MM, as well as Fast Track designations for AML, MPM, and MM from the FDA. In October 2024, the FDA granted Rare Pediatric Disease, or RPD, designation to GPS for the treatment of pediatric AML.

*SLS009: Highly Selective Next Generation CDK9 Inhibitor*

On March 31, 2022, we entered into an exclusive license agreement, or the GenFleet Agreement, with GenFleet Therapeutics (Shanghai), Inc., or GenFleet, a clinical-stage biotechnology company developing cutting-edge therapeutics in oncology and immunology, that grants rights to us for the development and commercialization of SLS009, a highly selective small molecule CDK9 inhibitor, across all therapeutic and diagnostic uses worldwide, except for Greater China.

CDK9 activity has been shown to correlate negatively with overall survival in a number of cancer types, including hematologic cancers, such as AML and lymphomas, as well as solid cancers, such as osteosarcoma, pediatric soft tissue sarcomas, melanoma, endometrial, lung, prostate, breast and ovarian. As demonstrated in preclinical and clinical data, to date, SLS009's high selectivity has the potential to reduce toxicity as compared to older CDK9 inhibitors and other next-generation CDK9 inhibitors currently in clinical development and to potentially be more efficacious.

We completed a Phase 1 dose-escalating clinical trial in the United States and China for SLS009 in mid-2023 and reported positive safety and efficacy data for both patient cohorts, that is relapsed and/or refractory AML and refractory lymphoma. We also established in the trial a recommended Phase 2 dose, or RP2D, of 60 mg once weekly for AML and 100 mg once weekly for lymphomas.

In the second quarter of 2023, we commenced an open label, single arm, multi-center Phase 2a clinical trial with SLS009 in combination with venetoclax and azacitidine, or aza/ven, in patients with AML who failed or did not respond to treatment with venetoclax-based therapies. The trial is evaluating safety, tolerability, and efficacy at two dose levels of SLS009, 45 mg once weekly, and 60 mg once weekly or 30 mg twice a week, in combination with aza/ven. In addition to safety and tolerability of SLS009 in combination with aza/ven, the efficacy endpoints are complete response composite rate and duration of response. Additional endpoints include event free survival, overall survival, and pharmacokinetic and pharmacodynamic assessments.

In the fourth quarter of 2023, we completed enrollment in the 45 mg (safety) dose cohort in the Phase 2a study and reported positive initial topline data. At that time, we also commenced enrollment in the 60 mg dose cohort with patients randomized to one of two groups, 60 mg fixed dose once weekly or 30 mg fixed twice weekly. Each group was planned to enroll five to 10 patients. During the trial we identified 30 mg fixed twice weekly as our optimal dose level.

During the trial, we identified potential biomarkers currently undergoing testing as predictive markers in the most recent portion of the study. In May 2024, we announced additional preliminary data from the Phase 2a trial of SLS009 in r/r AML and successful filing of a provisional patent application around the ASXL1 mutation and SLS009, including all CDK9 inhibitor drugs. ASXL1 mutations are associated with poor prognosis in all myeloid diseases, owing to the reduced response to the current treatment options. We observed a high rate of responses in patients with myelodysplasia-related molecular mutations, or AML MR, as defined by the World Health Organization, and patients with the ASXL1 gene mutation accounted for the most responders across all dose cohorts. We expanded the ongoing study to include two additional cohorts, one with ASXL1 mutated AML patients and one with patients with myelodysplasia-related molecular abnormalities other than ASXL1 at the optimal dose level of 30 mg fixed twice weekly. In December 2024, we announced positive data from the first 3 cohorts in the Phase 2a trial.

In July 2025, we announced that the Phase 2 trial of SLS009 in r/r AML met all primary endpoints and received FDA guidance to advance our first-line therapy study. The overall response rate, or ORR, in 54 evaluable patients was 33% across all cohorts and dose levels, 40% for the 30 mg BIW dose level, and 44% in the 30 mg BIW dose among AML-Myelodysplasia-Related Changes, or AML MR, patients, all exceeding the pre-specified ORR threshold of 20%. The highest efficacy was observed among patients with ASXL1 mutations, with an ORR of 50% (9/18) at 30 mg BIW dose levels, and AML MR with Myelomonocytic/Myelomonoblastic, or M4/M5, patients with an ORR of 50% (6/12). The median overall survival, or mOS, reached 8.9 months in patients with AML MR and 8.8 months in patients r/r to venetoclax-based regimens at a 30 mg BIW dose level, surpassing the historical benchmark of ~2.4 months. SLS009 was well-tolerated with no new safety signals observed. No dose-limiting toxicities were observed across all dose levels.

In addition, following a productive end of Phase 2 meeting, the FDA recommended that we proceed into a trial to include newly diagnosed, first-line AML patients eligible for aza/ven therapy, where the FDA noted clinical benefit might be greatest. The randomized 80-patient trial is currently in preparation and expected to begin enrollment by the first quarter of 2026. The trial will include two groups: predictive biomarker cohort (newly diagnosed patients unlikely to benefit from standard aza/ven therapy based on molecular profiling) and early resistance cohort (patients who initiate treatment with aza/ven, but demonstrate confirmed lack of any response after two treatment cycles).

In November 2024, we announced data from preclinical studies identifying ASXL1 mutation as key predictor of SLS009 in response to solid cancers. SLS009 is also currently being evaluated in pediatric solid tumors and leukemia models through the NCI Pediatric Preclinical in Vivo Testing, or PIVOT, program. Studies are supported through cooperative agreement grants from the NCI to the PIVOT research centers performing the testing in pediatric tumor models and a centralized coordinating center.

In May 2025, we announced data for pediatric acute lymphoblastic leukemia, or ALL, patients derived xenografts, or PDX. The experiment conducted and funded by the National Institute of Health, or NIH, through PIVOT program included 27 patient-derived ALL tumors from pediatric patients. Tumors were xenografted in mice in two groups, vehicle control arm and SLS009 arm. Mice were treated with a fractionated dose once per week for six consecutive weeks. Treatment was well tolerated. For all models, median survival was approximately tripled in the SLS009 arm, compared to vehicle control arm. SLS009 demonstrated delayed progression in 25/27 (93%) models and more than two times longer time to progression in 15/27 (56%) of ALL models. In addition, there were complete responses, or CR, in two models and in one of the two models CR was maintained after the treatment had been completed until the end of the study (four months). Among seven KMT2A rearranged models, time to progression was extended in all seven models, and in six out of seven (86%) time to progression was more than doubled.

For SLS009, the FDA granted Orphan Drug Product designations in AML and peripheral T-cell lymphoma, or PTCL, and Fast Track designations for r/r AML and r/r PTCL. The FDA granted RPD designation to SLS009 for the treatment of pediatric acute lymphoblastic leukemia, or ALL, in June 2024 and the FDA granted RPD designation to SLS009 for the treatment of pediatric AML in July 2024. Also, the European Medicines Agency granted Orphan Drug Designation for SLS009 in AML and in PTCL in June 2024 and July 2024, respectively.

**Recent Developments**

On September 10, 2025, we entered into a Warrant Inducement Agreement (the "Inducement Agreement") with a holder (the "Holder") of certain existing warrants (the "Existing Warrants") to purchase shares of Common Stock. Pursuant to the Inducement Agreement, the Holder and the Company agreed that, subject to any applicable beneficial ownership limitations, the Holder would cash exercise 19,685,040 shares of Common Stock at an exercise price of $1.20 per share, the original issuance exercise price of the Existing Warrants. The Company received aggregate gross proceeds of approximately $23.6 million from the exercise of the Existing Warrants before deducting financial advisory fees and other expenses payable by the Company.

In consideration of the Holder's agreement to exercise the Existing Warrants in accordance with the Inducement Agreement, the Company issued to the Holder new warrants (the "Inducement Warrants") to purchase up to 19,685,040 shares of Common Stock, which is equal to 100% of the number of shares of Common Stock issued upon exercise of the Existing Warrants (the "Inducement Warrant Shares"). The Company agreed in the Inducement Agreement to file a registration statement within 30 days of September 10, 2025, providing for the resale of the Inducement Warrant Shares by the Holder.

The Inducement Warrants have an exercise price of $1.88 per share, which was the closing price of the Common Stock on the Nasdaq Capital Market on September 10, 2025, are exercisable immediately and expire on the five and one half-year anniversary of issuance.

The Company engaged A.G.P./Alliance Global Partners ("A.G.P.") and Maxim Group LLC ("Maxim", and together with A.G.P., the "Financial Advisors") to act as Financial Advisors in connection with the transactions summarized above and paid the Financial Advisors an aggregate fee equal to 6.5% of the gross proceeds raised in the transaction.

**Corporate Information**

Our principal executive offices are located at 7 Times Square, Suite 2503, New York, NY 10036, and our phone number is (646) 200-5278. Our website address is www.sellaslifesciences.com. We do not incorporate the information on our website into this Registration Statement, and you should not consider such information part of this Registration Statement.

We were incorporated on April 3, 2006 in Delaware as Argonaut Pharmaceuticals, Inc. On November 28, 2006, we changed our name to RXi Pharmaceuticals Corporation and began operations January 2007. On September 26, 2011, we changed our name to Galena Biopharma, Inc., or Galena. In December 2017, we completed a business combination, or the Merger, with SELLAS Life Sciences Group, Ltd., a privately held Bermuda exempted company, or Private SELLAS, and changed our name to "SELLAS Life Sciences Group, Inc."

**Implications of Being a Smaller Reporting Company**

We are a "smaller reporting company" as defined in Item 10(f)(1) of Regulation S-K. Smaller reporting companies may take advantage of certain reduced disclosure obligations, including, among other things, providing only two years of audited financial statements. We will remain a smaller reporting company until the last day of any fiscal year for so long as either (1) the market value of our shares of Common Stock held by non-affiliates does not equal or exceed $250.0 million as of the prior June 30th, or (2) our annual revenues did not equal or exceed $100.0 million during such completed fiscal year and the market value of our shares of Common Stock held by non-affiliates did not equal or exceed $700.0 million as of the prior June 30th. To the extent we take advantage of any reduced disclosure obligations, it may make comparison of our financial statements with other public companies difficult or impossible.

**THE OFFERING**

---

| | |
|:---|:---|
| **Common stock offered by the selling stockholders** | Up to 19,685,040 shares of Common Stock underlying the Inducement Warrants. |
| **Terms of the offering** | The selling stockholders will determine when and how they will sell the Common Stock offered in this prospectus, as described in "Plan of Distribution." |
| **Use of proceeds** | We will not receive any proceeds from the sale of shares of our Common Stock by the selling stockholders. We may receive up to approximately $37,007,875.20 in aggregate gross proceeds from the exercise of the Inducement Warrants, if the Inducement Warrants are exercised for cash (and not a cashless basis), based on the per share exercise price of the Inducement Warrants. Any proceeds we receive from the exercise of the Inducement Warrants will be used for working capital and general corporate purposes. |
| **Risk factors** | An investment in our securities involves a high degree of risk. See the section entitled "Risk Factors" of this prospectus and the similarly titled sections in the documents incorporated by reference into this prospectus. |
| **Nasdaq Capital Market Symbol** | SLS |

---

The selling stockholders named in this prospectus may offer and sell up to 19,685,040 shares of our Common Stock.

The selling stockholders are prohibited, subject to certain exceptions, from exercising the Inducement Warrants to the extent that immediately prior to or after giving effect to such exercise, the selling stockholders, together with their respective affiliates and other attribution parties, would own more than 4.99% of the total number of shares of Common Stock then issued and outstanding, which percentage may be increased or decreased at the selling stockholder's election to a percentage not to exceed 9.99%, provided that any increase will require 61 days' notice to the Company.

Our Common Stock is currently listed on The Nasdaq Capital Market under the symbol "SLS."

Shares of Common Stock that may be offered under this prospectus will be fully paid and non-assessable. We will not receive any of the proceeds of sales by the selling stockholders of any of the Common Stock covered by this prospectus. Throughout this prospectus, when we refer to the shares of our Common Stock being registered on behalf of the selling stockholders for offer and resale, we are referring to the shares of Common Stock issued to the selling stockholders in connection with the exercise of warrants issued in the transaction as described above. When we refer to the selling stockholders in this prospectus, we are referring to the selling stockholders identified in this prospectus and, as applicable, their permitted transferees or other successors-in-interest that may be identified in a supplement to this prospectus or, if required, a post-effective amendment to the registration statement of which this prospectus is a part.

**RISK FACTORS**

Investing in our securities involves risks. Before you make a decision to buy our securities, in addition to the risks and uncertainties discussed above under "Special Note Regarding Forward-Looking Statements," you should carefully consider the specific risks incorporated by reference in this prospectus to our most recent Annual Report on Form 10-K and any subsequent Quarterly Reports on Form 10-Q or Current Reports on Form 8-K, and all other information contained or incorporated by reference into this prospectus, as updated by our subsequent filings under the Exchange Act, and the risk factors and other information contained in any applicable prospectus supplement and any applicable free writing prospectus before acquiring any such securities. If any of these risks actually occur, it may materially harm our business, financial condition, liquidity and results of operations. As a result, the market price of our securities could decline, and you could lose all or part of your investment. Additionally, the risks and uncertainties incorporated by reference in this prospectus or any prospectus supplement are not the only risks and uncertainties that we face. Additional risks and uncertainties not presently known to us or that we currently believe to be immaterial, but that could later become material, may also adversely affect our business.

**USE OF PROCEEDS**

We will not receive any of the proceeds from the sale of securities by the selling stockholder pursuant to this prospectus. We may receive up to approximately $37,007,875.20 in aggregate gross proceeds from the exercise of the Inducement Warrants, if the Inducement Warrants are exercised for cash (and not a cashless basis), based on the per share exercise price of the Inducement Warrants. Any proceeds we receive from the exercise of the Inducement Warrants will be used for working capital and general corporate purposes.

**SELLING STOCKHOLDERS**

The shares of Common Stock being offered by the selling stockholders are those issuable to the selling stockholders, upon exercise of the Inducement Warrants. For additional information regarding the issuances of those shares of Common Stock and the Inducement Warrants, see "Prospectus Summary-Recent Developments" above. We are registering the shares of Common Stock issuable upon exercise of the Inducement Warrants in order to permit the selling stockholders to offer the shares for resale from time to time. Except for the ownership of the shares of Common Stock and the Inducement Warrants, the selling stockholders have not had any material relationship with us within the past three years. The selling stockholders may sell all, some or none of their shares in this offering. See "Plan of Distribution."

The table below lists the selling stockholders and other information regarding the beneficial ownership (as determined under Section 13(d) of the Exchange Act and the rules and regulations thereunder) of the shares of Common Stock by each of the selling stockholders. Under the terms of the Inducement Warrants, a selling stockholder may not exercise the Inducement Warrants to the extent such exercise would cause such selling stockholder, together with its affiliates, to beneficially own a number of shares of Common Stock that would exceed 4.99% of our then outstanding shares of Common Stock following such exercise, which percentage may be increased or decreased at the selling stockholder's election to a percentage not to exceed 9.99%, provided that any increase will require 61 days' notice to the Company, excluding for purposes of such determination shares of Common Stock issuable upon exercise of the Inducement Warrants which have not been exercised. The number of shares beneficially owned prior to the offering in the second column (i) is based on 125,078,525 shares of our Common Stock outstanding as of October 1, 2025, and (ii) takes into account the beneficial ownership limitations described above.

The third column lists the shares of Common Stock being offered by this prospectus by the selling stockholders. In accordance with the terms of our registration rights agreement with the selling stockholders, this prospectus generally covers the resale of the maximum number of shares of Common Stock issuable upon exercise of the Inducement Warrants, without regard to any limitations on the exercise of the Inducement Warrants, and this registration statement registers the maximum number of shares of Common Stock as shall from time to time be necessary to effect the exercise of the Inducement Warrants then outstanding without giving effect to any limitation on exercise included in the Inducement Warrants. The number and percentage of shares beneficially owned after the offering in the fourth and fifth columns (i) is based on 125,078,525 shares of our Common Stock outstanding as of October 1, 2025, (ii) assumes the sale of all of the shares offered by the selling stockholders pursuant to this prospectus and (iii) takes into account the beneficial ownership limitations described above. This table was prepared based on information furnished to us by the selling stockholders.

---

| | | | | | |
|:---|:---|:---|:---|:---|:---|
| **Name of Selling<br> Stockholder** | **Number of Shares of<br> Common Stock<br> Owned Prior to<br> Offering** |  | **Maximum Number<br> of Shares of<br> Common Stock to be<br> Sold Pursuant to this <br> Prospectus** | **Number of Shares<br> of Common Stock <br> Owned After<br> Offering(1)** | **Percentage of Shares <br> of Common Stock<br> Owned After<br> Offering if Greater<br> than 1%** |
| Anson Investments Master Fund LP | 37478027 | (2) | 15354331 | 22123696 | 4.99% |
| Anson East Master Fund LP | 10570727 | (3) | 4330709 | 6240018 | 4.80% |

---

(1) Assumes that all Common Stock being registered under the registration statement of which this prospectus
forms a part are sold in this offering, and that none of the selling stockholders acquire additional Common Stock after the date of this
prospectus and prior to completion of this offering.

(2) Consists of (i) 15,354,331 shares of Common Stock being registered hereby, (ii) 4,680,000 shares
of Common Stock held as of October 1, 2025 and (iii) 17,443,696 shares of Common Stock issuable upon the exercise of previously
held warrants. Anson Advisors Inc and Anson Funds Management LP, the Co-Investment Advisers of Anson Investments Master Fund LP ("Anson"),
hold voting and dispositive power over the shares of Common Stock held by Anson. Tony Moore is the managing member of Anson Management
GP LLC, which is the general partner of Anson Funds Management LP. Moez Kassam and Amin Nathoo are directors of Anson Advisors Inc. Mr. Moore,
Mr. Kassam and Mr. Nathoo each disclaim beneficial ownership of these shares of Common Stock except to the extent of their pecuniary
interest therein. The principal business address of Anson is Maple Corporate Services Limited, PO Box 309, Ugland House, Grand Cayman,
KYI-1104, Cayman Islands.

(3) Consists of (i) 4,330,709 shares of Common Stock being registered hereby, (ii) 1,320,000 shares
of Common Stock held as of October 1, 2025 and (iii) 4,920,018 shares of Common Stock issuable upon the exercise of previously
held warrants. Anson Advisors Inc and Anson Funds Management LP, the Co-Investment Advisers of Anson East Master Fund LP ("Anson
East"), hold voting and dispositive power over the shares of Common Stock held by Anson East. Tony Moore is the managing member
of Anson Management GP LLC, which is the general partner of Anson Funds Management LP. Moez Kassam and Amin Nathoo are directors of Anson
Advisors Inc. Mr. Moore, Mr. Kassam and Mr. Nathoo each disclaim beneficial ownership of these shares of Common Stock except
to the extent of their pecuniary interest therein. The principal business address of Anson East is Maple Corporate Services Limited, PO
Box 309, Ugland House, Grand Cayman, KYI-1104, Cayman Islands.

**PLAN OF DISTRIBUTION**

We are registering the shares of Common Stock issuable upon exercise of the Inducement Warrants to permit the resale of these shares of Common Stock from time to time after the date of this prospectus. We will not receive any of the proceeds from the sale by the selling stockholders of the shares of Common Stock. We will bear all fees and expenses incident to our obligation to register the shares of Common Stock.

The selling stockholders may sell all or a portion of the shares of Common Stock they acquire upon exercise of the Inducement Warrants beneficially owned by them and offered hereby from time to time directly or through one or more underwriters, broker-dealers or agents. If the shares of Common Stock are sold through underwriters or broker-dealers, the selling stockholders will be responsible for underwriting discounts or commissions or agent's commissions. The shares of Common Stock may be sold in one or more transactions at fixed prices, at prevailing market prices at the time of the sale, at varying prices determined at the time of sale, or at negotiated prices. These sales may be effected in transactions, which may involve crosses or block transactions,

&nbsp;&nbsp;&nbsp;&nbsp;· on any national securities exchange or quotation
service on which the securities may be listed or quoted at the time of sale;

&nbsp;&nbsp;&nbsp;&nbsp;· in the over-the-counter market;

&nbsp;&nbsp;&nbsp;&nbsp;· in transactions otherwise than on these exchanges
or systems or in the over-the-counter market;

&nbsp;&nbsp;&nbsp;&nbsp;· through the writing of options, whether such
options are listed on an options exchange or otherwise;

&nbsp;&nbsp;&nbsp;&nbsp;· ordinary brokerage transactions and transactions
in which the broker-dealer solicits purchasers;

&nbsp;&nbsp;&nbsp;&nbsp;· block trades in which the broker-dealer will
attempt to sell the shares as agent but may position and resell a portion of the block as principal to facilitate the transaction;

&nbsp;&nbsp;&nbsp;&nbsp;· purchases by a broker-dealer as principal and
resale by the broker-dealer for its account;

&nbsp;&nbsp;&nbsp;&nbsp;· an exchange distribution in accordance with the
rules of the applicable exchange;

&nbsp;&nbsp;&nbsp;&nbsp;· privately negotiated transactions;

&nbsp;&nbsp;&nbsp;&nbsp;· short sales;

&nbsp;&nbsp;&nbsp;&nbsp;· sales pursuant to Rule 144;

&nbsp;&nbsp;&nbsp;&nbsp;· broker-dealers may agree with the selling stockholders
to sell a specified number of such shares at a stipulated price per share;

&nbsp;&nbsp;&nbsp;&nbsp;· a combination of any such methods of sale; and

&nbsp;&nbsp;&nbsp;&nbsp;· any other method permitted pursuant to applicable
law.

If the selling stockholders effect such transactions by selling shares of Common Stock to or through underwriters, broker-dealers or agents, such underwriters, broker-dealers or agents may receive commissions in the form of discounts, concessions or commissions from the selling stockholders or commissions from purchasers of the shares of Common Stock for whom they may act as agent or to whom they may sell as principal (which discounts, concessions or commissions as to particular underwriters, broker-dealers or agents may be in excess of those customary in the types of transactions involved). In connection with sales of the shares of Common Stock or otherwise, the selling stockholders may enter into hedging transactions with broker-dealers, which may in turn engage in short sales of the shares of Common Stock in the course of hedging in positions they assume. The selling stockholders may also sell shares of Common Stock short and deliver shares of Common Stock covered by this prospectus to close out short positions and to return borrowed shares in connection with such short sales. The selling stockholders may also loan or pledge shares of Common Stock to broker-dealers that in turn may sell such shares.

The selling stockholders may pledge or grant a security interest in some or all of the Inducement Warrants or shares of Common Stock they acquire upon the exercise of the Inducement Warrants owned by them and, if they default in the performance of their secured obligations, the pledgees or secured parties may offer and sell the shares of Common Stock from time to time pursuant to this prospectus or any amendment to this prospectus under Rule 424(b)(3) or other applicable provision of the Securities Act of 1933, as amended, amending, if necessary, the list of selling stockholders to include the pledgee, transferee or other successors in interest as selling stockholders under this prospectus. The selling stockholders also may transfer and donate the shares of Common Stock in other circumstances in which case the transferees, donees, pledgees or other successors in interest will be the selling beneficial owners for purposes of this prospectus.

The selling stockholders and any broker-dealer participating in the distribution of the shares of Common Stock may be deemed to be "underwriters" within the meaning of the Securities Act, and any commission paid, or any discounts or concessions allowed to, any such broker-dealer may be deemed to be underwriting commissions or discounts under the Securities Act. At the time a particular offering of the shares of Common Stock is made, a prospectus supplement, if required, will be distributed which will set forth the aggregate amount of shares of Common Stock being offered and the terms of the offering, including the name or names of any broker-dealers or agents, any discounts, commissions and other terms constituting compensation from the selling stockholders and any discounts, commissions or concessions allowed or reallowed or paid to broker-dealers.

Under the securities laws of some states, the shares of Common Stock may be sold in such states only through registered or licensed brokers or dealers. In addition, in some states the shares of Common Stock may not be sold unless such shares have been registered or qualified for sale in such state or an exemption from registration or qualification is available and is complied with.

There can be no assurance that any selling stockholder will sell any or all of the shares of Common Stock registered pursuant to the registration statement, of which this prospectus forms a part.

The selling stockholders and any other person participating in such distribution will be subject to applicable provisions of the Securities Exchange Act of 1934, as amended, and the rules and regulations thereunder, including, without limitation, Regulation M of the Exchange Act, which may limit the timing of purchases and sales of any of the shares of Common Stock by the selling stockholders and any other participating person. Regulation M may also restrict the ability of any person engaged in the distribution of the shares of Common Stock to engage in market-making activities with respect to the shares of Common Stock. All of the foregoing may affect the marketability of the shares of Common Stock and the ability of any person or entity to engage in market-making activities with respect to the shares of Common Stock.

We will pay all expenses of the registration of the shares of Common Stock issuable upon exercise of the Inducement Warrants, estimated to be $54,063.21 in total, including, without limitation, SEC filing fees and expenses of compliance with state securities or "blue sky" laws; provided, however, that a selling stockholder will pay all underwriting discounts and selling commissions, if any. We will indemnify the selling stockholders against liabilities, including some liabilities under the Securities Act, in accordance with the registration rights agreements, or the selling stockholders will be entitled to contribution. We may be indemnified by the selling stockholders against civil liabilities, including liabilities under the Securities Act, that may arise from any written information furnished to us by the selling stockholder specifically for use in this prospectus, in accordance with the related registration rights agreement, or we may be entitled to contribution.

Once sold under the registration statement, of which this prospectus forms a part, the shares of Common Stock will be freely tradable in the hands of persons other than our affiliates.

**LEGAL MATTERS**

The validity of the securities we are offering will be passed upon for us by Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C., New York, New York.

**EXPERTS**

The consolidated financial statements of the Company incorporated in this Prospectus by reference from the [Annual Report on Form 10-K of the Company for the year ended December 31, 2024](https://www.sec.gov/ix?doc=/Archives/edgar/data/1390478/000139047825000003/sls-20241231.htm), have been audited by Baker Tilly US, LLP (formerly, Moss Adams LLP), an independent registered public accounting firm, as stated in their report (which report expresses an unqualified opinion and includes an explanatory paragraph related to a going concern uncertainty), which is incorporated herein by reference. Such consolidated financial statements are incorporated by reference in reliance upon the report of such firm given their authority as experts in accounting and auditing.

**WHERE YOU CAN FIND MORE INFORMATION**

We have filed a registration statement on Form S-3, including exhibits, under the Securities Act with respect to the securities offered by this prospectus. This prospectus does not contain all of the information included in the registration statement. For further information pertaining to us and our securities, you should refer to the registration statement and our exhibits.

In addition, we file annual, quarterly and current reports, proxy statements and other information with the SEC. Our filings with the SEC are available to the public on a website maintained by the SEC located at www.sec.gov. We also maintain a website at www.sellaslifesciences.com. Through our website, we make available, free of charge, annual, quarterly and current reports, proxy statements and other information as soon as reasonably practicable after they are electronically filed with, or furnished to, the SEC. The information contained on, or that may be accessed through, our website is not part of, and is not incorporated into, this prospectus. We include our website address in this prospectus only as an inactive textual reference. Information contained in our website does not constitute a part of this prospectus or our other filings with the SEC.

**INCORPORATION OF CERTAIN INFORMATION BY REFERENCE**

The SEC allows us to "incorporate by reference" information that we file with them. Incorporation by reference allows us to disclose important information to you by referring you to those other documents. The information incorporated by reference is an important part of this prospectus, and information that we file later with the SEC will automatically update and supersede this information. We filed a registration statement on Form S-3 under the Securities Act with the SEC with respect to the securities we may offer pursuant to this prospectus. This prospectus omits certain information contained in the registration statement, as permitted by the SEC. You should refer to the registration statement, including the exhibits, for further information about us and the securities we may offer pursuant to this prospectus. Statements in this prospectus regarding the provisions of certain documents filed with, or ,incorporated by reference in, the registration statement are not necessarily complete and each statement is qualified in all respects by that reference. Copies of all or any part of the registration statement, including the documents incorporated by reference or the exhibits, may be obtained upon payment of the prescribed rates at the offices of the SEC listed above in "Where You Can Find More Information." The documents we are incorporating by reference are:

· our
 Annual Report on Form 10-K for the year ended December 31, 2024, filed with the
 SEC on [March 20, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/0001390478/000139047825000003/sls-20241231.htm) ;

· our
 Quarterly Reports on Form 10-Q for the fiscal quarters ended March 31, 2025 and
 June 30, 2025, filed with the SEC on [May 13, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/1390478/000139047825000008/sls-20250331.htm) and [August 12, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/0001390478/000139047825000016/sls-20250630.htm) respectively;

· our
 Current Reports on Form 8-K filed with the SEC on [January 16, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/0001390478/000110465925004107/tm253479d1_8k.htm) , [January 29, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/0001390478/000110465925007049/tm254790d1_8k.htm) , [June 6, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/0001390478/000110465925057308/tm2517225d1_8k.htm) , [June 18, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/0001390478/000110465925060704/tm2518346d1_8k.htm) and [September 11, 2025](https://www.sec.gov/ix?doc=/Archives/edgar/data/0001390478/000110465925089272/tm2525854d1_8k.htm) , except for the
 information furnished under Items 2.02 or 7.01 and the exhibits thereto;

· the
 description of our common stock set forth in our registration statement on [Form 8-A, filed with the SEC on February 8, 2008](https://www.sec.gov/Archives/edgar/data/1390478/000095013508000652/b684708ae8va12b.htm) , [as amended on February 12, 2008](https://www.sec.gov/Archives/edgar/data/1390478/000095013508000827/b68470aae8va12bza.htm) ,
 including any further amendments thereto or reports filed for the purposes of updating this
 description; and

· all
 documents filed by us with the SEC pursuant to Sections 13(a), 13(c), 14 and 15(d) of
 the Exchange Act (other than Current Reports furnished under Item 2.02 or Item 7.01 of Form 8-K
 and exhibits filed on such forms that are related to such items) after the date of this prospectus
 and prior to the termination of this offering, provided that all documents "furnished"
 by the Company to the SEC and not "filed" are not deemed incorporated by reference
 herein.

In addition, all reports and other documents filed by us pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act after the date of the initial registration statement and prior to effectiveness of the registration statement shall be deemed to be incorporated by reference into this prospectus and deemed to be a part of this prospectus from the date of filing of such reports and documents. Notwithstanding the foregoing, we are not incorporating by reference any information furnished pursuant to Items 2.02 or 7.01 and the exhibits filed on such forms that are related to such items.

Any statement contained in a document incorporated by reference herein shall be deemed to be modified or superseded for all purposes to the extent that a statement contained in this prospectus or in any other subsequently filed document which is also incorporated or deemed to be incorporated by reference, modifies or supersedes such statement. Any statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this prospectus. You may request a copy of these filings (other than an exhibit to a filing unless that exhibit is specifically incorporated by reference into that filing) at no cost by writing, telephoning or e-mailing us at the following address or telephone number:

SELLAS Life Sciences Group, Inc.,

Attention: Corporate Secretary

7 Times Square

Suite 2503

New York, New York 10036

(646) 200-5278

You should rely only on the information incorporated by reference or provided in this prospectus or any prospectus supplement. We have not authorized anyone to provide you with different information. You should not assume that the information contained in this prospectus or the accompanying prospectus supplement is accurate on any date subsequent to the date set forth on the front of the document or that any information we have incorporated by reference is correct on any date subsequent to the date of the document incorporated by reference, even though this prospectus and any accompanying prospectus supplement is delivered or securities are sold on a later date.

![](tm2528102d1_s3img001.jpg)

**SELLAS Life Sciences Group, Inc.**

**19,685,040 Shares of Common Stock**

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;**, 2025**

**PART II**

**INFORMATION NOT REQUIRED IN PROSPECTUS**

**Item 14. Other Expenses of Issuance and Distribution**

The following table sets forth the costs and expenses payable in connection with the sale and distribution of the securities being registered. All amounts are estimates except the Securities and Exchange Commission (the "SEC") registration fee.

---

| | |
|:---|:---|
| SEC Registration Fee | $5063.21 |
| Legal Fees and Expenses | $35000 |
| Accounting Fees and Expenses | $9000 |
| Miscellaneous | $5000 |
| **Total** | $54063.21 |

---

**Item 15. Indemnification of Directors and Officers**

As permitted by Section 102 of the DGCL, we have adopted provisions in our amended and restated certificate of incorporation and amended and restated bylaws, each as amended, which limit or eliminate the personal liability of our directors for a breach of their fiduciary duty of care as a director. The duty of care generally requires that, when acting on behalf of the corporation, directors exercise an informed business judgment based on all material information reasonably available to them. Consequently, a director will not be personally liable to us or our stockholders for monetary damages for breach of fiduciary duty as a director, except for liability for:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;· any breach of the director's duty of loyalty to us or our stockholders;

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;· any act or omission not in good faith or that involves intentional misconduct or a knowing violation of law;

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;· any act related to unlawful stock repurchases, redemptions or other distributions or payment of dividends; or

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;· any transaction from which the director derived an improper personal benefit.

These limitations of liability do not affect the availability of equitable remedies such as injunctive relief or rescission. Our amended and restated certificate of incorporation also authorizes us to indemnify our officers, directors and other agents to the fullest extent permitted under Delaware law.

As permitted by Section 145 of the DGCL, our amended and restated bylaws, as amended, provide that:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;· we may indemnify our directors, officers and employees to the fullest extent permitted by the DGCL, subject to limited exceptions;

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;· we may advance expenses to our directors, officers and employees in connection with a legal proceeding to the fullest extent permitted by the DGCL, subject to limited exceptions; and

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;· the rights provided in our amended and restated bylaws are not exclusive.

Our amended and restated certificate of incorporation and bylaws, each as amended, provide for the indemnification provisions described above and elsewhere herein. We have entered into separate indemnification agreements with our directors and officers that may be broader than the specific indemnification provisions contained in the DGCL. These indemnification agreements generally require us, among other things, to indemnify our officers and directors against liabilities that may arise by reason of their status or service as directors or officers, other than liabilities arising from willful misconduct. These indemnification agreements also generally require us to advance any expenses incurred by the directors or officers as a result of any proceeding against them as to which they could be indemnified. In addition, we have purchased a policy of directors' and officers' liability insurance that insures our directors and officers against the cost of defense, settlement or payment of a judgment in some circumstances. These indemnification provisions and the indemnification agreements may be sufficiently broad to permit indemnification of our officers and directors for liabilities, including reimbursement of expenses incurred, arising under the Securities Act.

We have entered into indemnification agreements with our directors and executive officers, in addition to the indemnification provided for in our amended and restated certificate of incorporation and amended and restated bylaws, and intend to enter into indemnification agreements with any new directors and executive officers in the future.

We have purchased and currently intend to maintain insurance on behalf of each and every person who is or was a director or officer of our company against any loss arising from any claim asserted against him or her and incurred by him or her in any such capacity, subject to certain exclusions.

**Item 16. Exhibits**

---

| | |
|:---|:---|
| **Exhibit<br> Number** | **Description** |
| [4.1](http://www.sec.gov/Archives/edgar/data/0001390478/000110465925089272/tm2525854d1_ex4-1.htm) | [Form of Inducement Warrant (incorporated by reference to Exhibit 4.1 to our Current Report on Form 8-K, filed on September 11, 2025).](http://www.sec.gov/Archives/edgar/data/0001390478/000110465925089272/tm2525854d1_ex4-1.htm) |
| [5.1\*](tm2528102d1_ex5-1.htm) | [Opinion of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.](tm2528102d1_ex5-1.htm) |
| [10.1](http://www.sec.gov/Archives/edgar/data/0001390478/000110465925089272/tm2525854d1_ex10-1.htm) | [Form of Inducement Agreement (incorporated by reference to Exhibit 10.1 to our Current Report on Form 8-K, filed on September 11, 2025).](http://www.sec.gov/Archives/edgar/data/0001390478/000110465925089272/tm2525854d1_ex10-1.htm) |
| [23.1\*](tm2528102d1_ex23-1.htm) | [Consent of Baker Tilly US, LLP, Independent Registered Public Accounting Firm](tm2528102d1_ex23-1.htm) |
| [23.2\*](tm2528102d1_ex5-1.htm) | [Consent of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. (contained in Exhibit 5.1 hereto).](tm2528102d1_ex5-1.htm) |
| [24.1\*](#a_001) | [Powers of Attorney (included in the signature page of this registration statement).](#a_001) |
| [107\*](tm2528102d1_ex-filingfees.htm) | [Filing Fee Table](tm2528102d1_ex-filingfees.htm) |

---

\* Filed herewith.

**Item 17. Undertakings**

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(a) The undersigned registrant hereby undertakes:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(1) To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(i) To include any prospectus required by section 10(a)(3) of the Securities Act of 1933;

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(ii) To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) if, in the aggregate, the changes in volume and price represent no more than a 20% change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement; and

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(iii) To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement;

*provided, however,* that paragraphs (a)(1)(i), (a)(1)(ii) and (a)(1)(iii) do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the registrant pursuant to section 13 or section 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the registration statement, or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(2) That, for the purpose of determining any liability under the Securities Act of 1933, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(3) To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(4) That, for the purpose of determining liability under the Securities Act of 1933 to any purchaser:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(i) Each prospectus filed by the registrant pursuant to Rule 424(b)(3) shall be deemed to be part of the registration statement as of the date the filed prospectus was deemed part of and included in the registration statement; and

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(ii) Each prospectus required to be filed pursuant to Rule 424(b)(2), (b)(5), or (b)(7) as part of a registration statement in reliance on Rule 430B relating to an offering made pursuant to Rule 415(a)(1)(i), (vii), or (x) for the purpose of providing the information required by section 10(a) of the Securities Act of 1933 shall be deemed to be part of and included in the registration statement as of the earlier of the date such form of prospectus is first used after effectiveness or the date of the first contract of sale of securities in the offering described in the prospectus. As provided in Rule 430B, for liability purposes of the issuer and any person that is at that date an underwriter, such date shall be deemed to be a new effective date of the registration statement relating to the securities in the registration statement to which that prospectus relates, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof. Provided, however, that no statement made in a registration statement or prospectus that is part of the registration statement or made in a document incorporated or deemed incorporated by reference into the registration statement or prospectus that is part of the registration statement will, as to a purchaser with a time of contract of sale prior to such effective date, supersede or modify any statement that was made in the registration statement or prospectus that was part of the registration statement or made in any such document immediately prior to such effective date.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(5) That, for the purpose of determining liability of the registrant under the Securities Act to any purchaser in the initial distribution of the securities, the undersigned registrant undertakes that in a primary offering of securities of the undersigned registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(i) Any preliminary prospectus or prospectus of the undersigned registrant relating to the offering required to be filed pursuant to Rule 424;

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(ii) Any free writing prospectus relating to the offering prepared by or on behalf of the undersigned registrant or used or referred to by the undersigned registrant;

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(iii) The portion of any other free writing prospectus relating to the offering containing material information about the undersigned registrant or its securities provided by or on behalf of the undersigned registrant; and

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(iv) Any other communication that is an offer in the offering made by the undersigned registrant to the purchaser.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(b) The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act of 1933, each filing of the registrant's annual report pursuant to section 13(a) or section 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan's annual report pursuant to section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(c) Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the registrant, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Act and will be governed by the final adjudication of such issue.

**SIGNATURES**

Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the city of New York, state of New York, on October 10, 2025.

---

| | |
|:---|:---|
| **SELLAS LIFE SCIENCES GROUP, INC.** | **SELLAS LIFE SCIENCES GROUP, INC.** |
| By: | */s/ Angelos M. Stergiou* |
|  | Angelos M. Stergiou, M.D., ScD h.c |
|  | President, Chief Executive Officer and Director |

---

**KNOW ALL PERSONS BY THESE PRESENTS,** that each person whose signature appears below hereby severally constitutes and appoints Angelos M. Stergiou and John T. Burns, and each of them singly, as such person's true and lawful attorneys-in-fact and agents, with full power of substitution and resubstitution, for such person and in such person's name, place and stead, in any and all capacities, to sign any and all amendments (including, without limitation, post-effective amendments) to this registration statement (or any registration statement for the same offering that is to be effective upon filing pursuant to Rule 462(b) under the Securities Act of 1933), and to file the same, with all exhibits thereto, and all documents in connection therewith, with the Securities and Exchange Commission, granting unto each said attorney-in-fact and agent, full power and authority to do and perform each and every act and thing requisite and necessary to be done in and about the premises, as fully to all intents and purposes as such person might or could do in person, hereby ratifying and confirming all that any said attorney-in-fact and agent, or any substitute or substitutes of any of them, may lawfully do or cause to be done by virtue hereof.

Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed by the following persons in the capacities and on the dates indicated.

---

| | | |
|:---|:---|:---|
| **Signature** | **Title** | **Date** |
| /s/ Angelos M. Stergiou | President, Chief Executive Officer and Director | October 10, 2025 |
| Angelos M. Stergiou, M.D., ScD h.c. | *(Principal Executive Officer)* |  |
| /s/ John T. Burns | Senior Vice President, Chief Financial Officer | October 10, 2025 |
| John T. Burns | *(Principal Financial and Accounting Officer)* |  |
| /s/ John Varian | Chair of the Board of Directors | October 10, 2025 |
| John Varian |  |  |
| /s/ David L. Scheinberg | Director | October 10, 2025 |
| David L. Scheinberg, M.D., Ph.D. |  |  |
| /s/ Robert Van Nostrand | Director | October 10, 2025 |
| Robert Van Nostrand |  |  |
| /s/ Jane Wasman | Director | October 10, 2025 |
| Jane Wasman |  |  |
| /s/ Katherine Bach Kalin | Director | October 10, 2025 |
| Katherine Bach Kalin |  |  |

---

## Exhibit 5.1

**Exhibit 5.1**

---

| | |
|:---|:---|
| ![](tm2528102d1_ex5-1img01.jpg) | 919 Third Avenue<br> New York, NY 10022<br> 212 935 3000<br> mintz.com |

---

October 10, 2025

SELLAS Life Sciences Group, Inc.

7 Times Square, Suite 2503

New York, New York 10036

Ladies and Gentlemen:

We have acted as legal counsel to SELLAS Life Sciences Group, Inc., a Delaware corporation (the "<u>Company</u>"), in connection with the preparation and filing with the Securities and Exchange Commission (the "<u>Commission</u>") of a Registration Statement on Form S-3 (the "<u>Registration Statement</u>") on October 10, 2025 under the Securities Act of 1933, as amended (the "<u>Securities Act</u>"). The Registration Statement relates to the registration for resale, on a delayed or continuous basis, by the selling stockholders named in the Registration Statement (the "<u>Selling Stockholders</u>") of up to an aggregate of 19,685,040 shares (the "<u>Warrant Shares</u>") of the Company's common stock, $0.0001 par value per share ("<u>Common Stock</u>"), issuable upon the exercise of certain outstanding warrants (the "<u>Warrants</u>"). This opinion is being rendered in connection with the filing of the Registration Statement with the Commission.

In connection with this opinion, we have examined and relied upon the Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws of the Company, each as restated and/or amended to date, and such other corporate records, documents and agreements as we have considered necessary or appropriate for the purpose of rendering the opinions hereinafter set forth. In our examination, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals and the conformity with the originals of all documents submitted to us as copies. Upon the basis of such examination, we advise you that in our opinion that the Warrant Shares, when issued upon exercise of the Warrants in accordance with the terms thereof, will be validly issued, fully paid and non-assessable.

Our opinion is limited to the General Corporation Law of the State of Delaware, and we express no opinion with respect to the laws of any other jurisdiction. No opinion is expressed herein with respect to the qualification of the Warrant Shares under the securities or blue sky laws of any state or any foreign jurisdiction.

Please note that we are opining only as to the matters expressly set forth herein, and no opinion should be inferred as to any other matters. This opinion is based upon currently existing statutes, rules, regulations and judicial decisions, and we disclaim any obligation to advise you of any change in any of these sources of law or subsequent legal or factual developments which might affect any matters or opinions set forth herein.

We understand that you wish to file this opinion with the Commission as an exhibit to the Registration Statement in accordance with the requirements of Item 601(b)(5) of Regulation S-K promulgated under the Securities Act and to reference the firm's name under the caption "Legal Matters" in the prospectus which forms part of the Registration Statement, and we hereby consent thereto. In giving this consent, we do not admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

---

| |
|:---|
| Very truly yours, |
| /s/ Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. |
| Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C. |

---

<br> BOSTON LOS ANGELES NEW YORK SAN DIEGO SAN FRANCISCO TORONTO WASHINGTON

MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.

## Exhibit 23.1

**Exhibit 23.1**

**Consent of Independent Registered Public Accounting Firm**

We consent to the incorporation by reference in this Registration Statement on Form S-3 of SELLAS Life Sciences Group, Inc. (the "Company"), of our report dated March 20, 2025, relating to the consolidated financial statements of the Company (which report expresses an unqualified opinion and includes an explanatory paragraph relating to a going concern uncertainty), appearing in the Annual Report on Form 10-K of the Company for the year ended December 31, 2024, filed with the Securities and Exchange Commission. We also consent to the reference to us under the heading "Experts" in such Registration Statement.

/s/ Baker Tilly US, LLP

(formerly, Moss Adams LLP)

Campbell, California

October 10, 2025

## Ex-Filing

?xml version='1.0' encoding='ASCII'? EX-FILING FEES

---

| |
|:---|
| &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Calculation of Filing Fee Tables**  |
| &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **S-3**  |
| &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **SELLAS Life Sciences Group, Inc.**  |

---

---

| | | | | | | | | | |
|:---|:---|:---|:---|:---|:---|:---|:---|:---|:---|
| | | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Security Type**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Security Class Title**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Fee Calculation or Carry Forward Rule**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Amount Registered**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Proposed Maximum Offering Price Per Unit**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Maximum Aggregate Offering Price**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Fee Rate**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Amount of Registration Fee**  |
| **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** | **Newly Registered Securities** |
| Fees to be Paid | 1 | Equity | Common stock, $0.0001 par value per share, underlying Investor Warrants | 457(a) | 19685040 | $1.8625 | $36663387.00 | 0.0001381 | $5063.21 |
| Fees Previously Paid |  |  |  |  |  |  |  |  |  |
| **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** | **Carry Forward Securities** |
| Carry Forward Securities |  |  |  |  |  |  |  |  |  |
|  |  |  | Total Offering Amounts: | Total Offering Amounts: | Total Offering Amounts: |  | $36663387.00  |  | $5063.21  |
|  |  |  | Total Fees Previously Paid:  | Total Fees Previously Paid:  | Total Fees Previously Paid:  |  |  |  | $0.00  |
|  |  |  | Total Fee Offsets:  | Total Fee Offsets:  | Total Fee Offsets:  |  |  |  | $0.00  |
|  |  |  | Net Fee Due:  | Net Fee Due:  | Net Fee Due:  |  |  |  | $5063.21  |

---

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Offering Note** <br>

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <sup>1</sup> (1) This Registration Statement registers 19,685,040 shares of common stock, par value $0.0001 per share (the "Common Stock"), of SELLAS Life Sciences Group, Inc. (the "Company"), issuable upon the exercise of warrants (the "Warrants") that were sold in a private placement. (2) Estimated in accordance with Rule 457(c) solely for purposes of calculating the registration fee on the basis of the average of the high and low prices of the Common Stock as reported on The Nasdaq Capital Market on October 7, 2025, a date within five business days prior to filing this Registration Statement.

---

| | |
|:---|:---|
| | |
| **Rules 457(b) and 0-11(a)(2)** | **Rules 457(b) and 0-11(a)(2)** |
| Fee Offset Claims | N/A |
| Fee Offset Sources | N/A |
| **Rule 457(p)** | **Rule 457(p)** |
| Fee Offset Claims | N/A |
| Fee Offset Sources | N/A |

---

---

| | | | | | | | |
|:---|:---|:---|:---|:---|:---|:---|:---|
| | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Security Type**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Security Class Title**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Amount of Securities Previously Registered**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Maximum Aggregate Offering Price of Securities Previously Registered**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Form Type**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **File Number**  | &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; **Initial Effective Date**  |
| N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A |

---