This book constitutes the refereed proceedings of the 21st International Colloquium on Theoretical Aspects of Computing, ICTAC 2024, held in Bangkok, Thailand, during November 25-29, 2024. The 21 full papers presented here were carefully reviewed and selected from 46 submissions. They were categorized into the following topical sections: Invited Papers; Automata, Languages, and Learning; Algorithms and Complexity; Graphs and Games; Process Calculi; Verification and Reasoning.