This open access book constitutes the proceedings of the 26th International Conference on Agile Software Development, XP 2025, which took place in Brugg-Windisch, Switzerland, during June 2-5, 2025. XP is the premier agile software development conference combining research and practice. It is a unique forum where agile researchers, practitioners, thought leaders, coaches, and trainers get together to present and discuss their most recent innovations, research results, experiences, concerns, challenges, and trends.  XP conferences provide an informal environment to learn and trigger discussions and welcome both people new to agile and seasoned agile practitioners. The theme for 2025 was "Adapt - Uncovering better ways to deliver valuable software products". The 13 full papers and 4 short papers included in the proceedings were carefully reviewed and selected from 46 submissions. They were organized in topical sections as follows: Leadership and culture; business agility; engineering; and product and design.