This book constitutes the refereed proceedings of the 21st International Workshop on OpenMP, IWOMP 2025, in Charlotte, NC, USA, during October 1 – October 3, 2025

The 14 full papers included in this book were carefully reviewed and selected from 19 submissions. They were organized in topical sections as follows:New Directions: Communication and I/O; New Directions: Handling New Constraints and Devices; New Performance and Correctness Tooling**;** Advanced Capability Evaluation and New Directions: Programming Advances.