Company: PRTA
Filing Date: 2025-11-06
Form Type: DEFA14A
Source: 0001193125-25-269620
Chunk: 0

Company: PROTHENA CORP PUBLIC LTD CO
Filing Date: 2025-11-06
Form: DEFA14A
Chunk 0
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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 SCHEDULE 14A Proxy Statement Pursuant to Section 14(a) of the Securities Exchange Act of 1934 Filed by the Registrant ☒ Filed by a Party other than the Registrant ☐ Check the appropriate box:

| ☐ | Preliminary Proxy Statement |

| ☐ | Confidential, for Use of the Commission Only (as permitted by Rule 
 14a-6(e)(2))                                                       |

| ☐ | Definitive Proxy Statement |

| ☒ | Definitive Additional Materials |

| ☐ | Soliciting Material Pursuant to § 240.14a-12 |

PROTHENA CORPORATION PUBLIC LIMITED COMPANY (Name of registrant as specified in its charter) (Name of Person(s) Filing Proxy Statement, if other than the Registrant) Payment of Filing Fee (Check all boxes that apply):

| ☒ | No fee required. |

| ☐ | Fee paid previously with preliminary materials. |

| ☐ | Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 
 14a-6(i)(1) and 0-11                                                           |

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 8-K CURRENT REPORT Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934 Date of Report (Date of earliest event reported): November 6, 2025 PROTHENA CORPORATION PUBLIC LIMITED COMPANY (Exact name of registrant as specified in its charter)

| Ireland                      |     | 001-35676   |     | 98-1111119         |
| (State or Other Jurisdiction 
 of Incorporation)            |     | -Commission 
 File Number |     | -IRS Employer      
 Identification No. |

77 Sir John Rogerson’s Quay, Block C Grand Canal Docklands Dublin 2, D02 VK60, Ireland (Address of principal executive offices, including Zip Code) Registrant’s telephone number, including area code: 011-353-1-236-2500 (Former Name or Former Address, if Changed Since Last Report.) Check the appropriate box below if the Form 8-Kfiling is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions ( seeGeneral Instruction A.2. below):

| ☐ | Written communications pursuant to Rule 425 under the Securities