This open access book constitutes the proceedings of the 27th International Conference on Agile Software Development, XP 2026, which took place in São Paulo, Brazil, during April 8-11, 2026. 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 2026 was “Expanding Agile: Transforming Today, Shaping a Sustainable and Inclusive Tomorrow”.The 16 full papers and 2 short papers included in the proceedings were carefully reviewed and selected from 52 submissions. They were organized in topicsl sections as follows: Leadership and Culture; Business Agility; Data-Driven Agility and Transformations; Engineering; AI Augmented Software Engineering; Product and Design; and AI in Agile Teams.