The four-volume set, LNCS 12825, LNCS 12826, LNCS 12827, and LNCS 12828, constitutes the refereed proceedings of the 41st Annual International Cryptology Conference, CRYPTO 2021. Crypto has traditionally been held at UCSB every year, but due to the COVID-19 pandemic it was an online event in 2021.The 103 full papers presented in the proceedings were carefully reviewed and selected from a total of 426 submissions. The papers are organized in the following topical sections:Part I: Award Papers; Signatures; Quantum Cryptography; Succinct Arguments.Part II: Multi-Party Computation; Lattice Cryptography; and Lattice Cryptanalysis.Part III: Models; Applied Cryptography and Side Channels; Cryptanalysis; Codes and Extractors; Secret Sharing.Part IV: Zero Knowledge; Encryption++; Foundations; Low-Complexity Cryptography; Protocols.