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.