This book constitutes the proceedings of the 21st International Conference on Unconventional Computation and Natural Computation, UCNC 2024, held in Pohang, South Korea, during June 17-21, 2024. The 19 full papers included in this book were carefully reviewed and selected from 27 submissions. Typical, but not exclusive, UCNC topics of interest include amorphous computing, cellular automata, chaos and dynamical systems-based computing, cellular, chemical, evolutionary, bacterial, molecular, neural and optical computing, collision-based computing, quantum computing, DNA computing, membrane computing, material computing and programmable matter, super-Turing computation, swarm intelligence, and other nature-inspired algorithms.