This book constitutes the refereed proceedings of the 27th IFIP WG 1.02 International Conference on Descriptional Complexity of Formal Systems, DCFS 2026, held in Kingston, ON, Canada, during August 9–11, 2026.The 13 full papers included in this book were carefully reviewed and selected from 14 submissions. They focus on the Descriptional Complexity of Automata, Grammars, and Related Structures, Formal Descriptions and Software Reliability.