This book constitutes the proceedings of the 23rd International Conference on Descriptional Complexity of Format Systems, DCFS 2021, which was supposed to take place in Seoul South Korea in June 2021, but the conference was canceled due to the COVID-19 crisis.The 16 full papers presented in this volume were carefully reviewed and selected from 21 submissions. The conference focus to all aspects of descriptional complexity-the costs of description of objects in various computational models such as Turing machines, pushdown automata, finite automata or grammars.