This book constitutes the refereed proceedings of the 15th International Conference on Integrated Formal Methods, IFM 2019, held in Bergen, Norway, in December 2019. The 25 full papers and 3 short papers were carefully reviewed and selected from 95 submissions. The papers cover a broad spectrum of topics: from language design to verification and analysis techniques, to supporting tools and their integration into software engineering practice including both theoretical approaches and practical implementations. Also included are the extended abstracts of 6 "journal-first" papers.