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.