This double volume constitutes the thoroughly refereed post-conference proceedings of the 25th International Conference on Financial Cryptography and Data Security, FC 2021, held online due to COVID-19, in March 2021.The 47 revised full papers and 4 short papers together with 3 as Systematization of Knowledge (SoK) papers were carefully selected and reviewed from 223 submissions. The accepted papers were organized according to their topics in 12 sessions: Smart Contracts, Anonymity and Privacy in Cryptocurrencies, Secure Multi-Party Computation, System and Application Security, Zero-Knowledge Proofs, Blockchain Protocols, Payment Channels, Mining, Scaling Blockchains, Authentication and Usability, Measurement, and Cryptography.