This book constitutes the thoroughly refereed post-conference proceedings of the 23rd International Conference on Financial Cryptography and Data Security, FC 2019, held in St. Kitts, St. Kitts and Nevis in February 2019. The 32 revised full papers and 7 short papers were carefully selected and reviewed from 179 submissions. The papers are grouped in the following topical sections: Cryptocurrency Cryptanalysis, Measurement, Payment Protocol Security, Multiparty Protocols, Off-Chain Mechanisms, Fraud Detection, Game Theory, IoT Security and much more.