This open access book constitutes the refereed proceedings of the 15th International Symposium on Frontiers of Combining Systems, FroCoS 2025, held in Reykjavik, Iceland, during September 29–October 1, 2025. The 21 full papers included in the proceedings were carefully reviewed and selected from 32 submissions. They were organized in topical sections as follows: Description logics; beyond classical logic; satisfiability modulo theories; term rewrite systems; theorem proving; specific reasoning procedures; and proof checking.