This book constitutes the refereed proceedings of the 15th International Conference on Provable Security, ProvSec 2021, held in Guangzhou, China, in November 2021. The 21 full papers presented were carefully reviewed and selected from 67 submissions. The papers focus on provable security as an essential tool for analyzing security of modern cryptographic primitives. They are divided in the following topical sections: Searchable Encryption, Key Exchange & Zero Knowledge Proof, Post Quantum Cryptography, Functional Encryption, Digital Signature, and Practical Security Protocols.