This book constitutes the refereed proceedings of the 21st International Conference on Persuasive Technology, PERSUASIVE 2026, held in Hakodate, Japan, during March 11–13, 2026.The 25 full papers and 8 short papers presented in this volume were carefully reviewed and selected from 104 submissions. They focus on all aspects of design, development, and evaluation of interactive systems aimed at influencing people’s attitudes and behaviors through persuasion.