This book constitutes the refereed proceedings of the 35th Annual IFIP WG 11.3 Conference on Data and Applications Security and Privacy, DBSec 2021, held in Calgary, Canada, in July 2021.* The 15 full papers and 8 short papers presented were carefully reviewed and selected from 45 submissions. The papers present high-quality original research from academia, industry, and government on theoretical and practical aspects of information security. They are organized in topical sections named differential privacy, cryptology, machine learning, access control and others. *The conference was held virtually due to the COVID-19 pandemic.