This book constitutes the refereed proceedings of the 9th International Workshop on Post-Quantum Cryptography, PQCrypto 2018, held in Fort Lauderdale, FL, USA, in April 2018. The 24 revised full papers presented were carefully reviewed and selected from 97 submissions. The papers are organized in topical sections on Code-based Cryptography; Cryptanalysis; Hash-based Cryptography; Isogenies in Cryptography; Lattice-based Cryptography; Multivariate Cryptography; Protocols; Quantum Algorithms. .