This book constitutes the proceedings of the 24th International Symposium on Fundamentals of Computation Theory, FCT 2023, held in Trier, Germany, in September 2023. The 30 full papers included in this volume were carefully reviewed and selected from 79 submissions. The papers cover topics of all aspects of theoretical computer science, in particular Fundamentals of Computation Theory, Approximation Algorithms, Computable Analysis and much more.