This book constitutes the proceedings of the 19th International Conference on Unconventional Computation and Natural Computation, UCNC 2021, held in Espoo, Finland, in October 2021. The 12 full papers presented were carefully reviewed and selected from 19 submissions. The UCNC conference series covers fundamental research into computation that goes beyond the standard Turing model, including both computational models and methods inspired by nature, and the computational characteristics natural processes.