This volume constitutes the proceedings of the 12th International Conference on post-quantum cryptography, PQCrypto 2021, held in Daejeon, South Korea in July 2021. The 25 full papers presented in this volume were carefully reviewed and selected from 65 submissions. They cover a broad spectrum of research within the conference's scope, including code-, hash-, isogeny-, and lattice-based cryptography, multivariate cryptography, and quantum cryptanalysis.