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.