# EDGAR Filing Document

**Accession Number:** 0002022626
**File Stem:** 0001193125-26-159033
**Filing Date:** 2026-4
**Character Count:** 1138
**Document Hash:** c098e94ab8a7824f118219747445dc63
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001193125-26-159033.hdr.sgml**: 20260416

**ACCESSION NUMBER**: 0001193125-26-159033

**CONFORMED SUBMISSION TYPE**: ARS

**PUBLIC DOCUMENT COUNT**: 1

**CONFORMED PERIOD OF REPORT**: 20251231

**FILED AS OF DATE**: 20260416

**DATE AS OF CHANGE**: 20260416

**EFFECTIVENESS DATE**: 20260416

**FILER**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Upstream Bio, Inc.
- **CENTRAL INDEX KEY:** 0002022626
- **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:** ARS
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 001-42366
- **FILM NUMBER:** 26867742

**BUSINESS ADDRESS:**
- **STREET 1:** 890 WINTER STREET, SUITE 200
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02451
- **BUSINESS PHONE:** 781-208-2466

**MAIL ADDRESS:**
- **STREET 1:** 890 WINTER STREET, SUITE 200
- **CITY:** WALTHAM
- **STATE:** MA
- **ZIP:** 02451

### Attached PDF Documents

**Attachment 1:** `upb_ars_2025_10-k.pdf`

_No text found in this document._