This open access book constitutes the proceedings of the 29th International Conference on Foundations of Software Science and Computation Structures, FoSSaCS 2026, which was held as part of the International Joint Conferences on Theory and Practice of Software, ETAPS 2026, in Turin, Italy, during April 11–16, 2026.The 30 papers included in these proceedings were carefully reviewed and selected from 104 submissions. They focus on foundational research in software science on theories and methods to support the analysis, integration, synthesis, transformation, and verification of programs and software systems.