This book constitutes the refereed proceedings of the 26th IFIP WG 1.02 International Conference on Descriptional Complexity of Formal Systems, DCFS 2025, held in Loughborough, UK, during July 22–24, 2025.This book includes 15 full papers, carefully reviewed and selected from 20 submissions, as well as 4 abstracts of the invited talks. The conference focus on all aspects of descriptional complexity, including automata, grammars, languages, and other formal systems; various modes of operations and complexity measures.