This book constitutes the proceedings of the 24th RoboCup International Symposium which was held online during June 22 - June 28, 2021. The 19 full papers included in these proceedings were carefully reviewed and selected from 42 submissions; the volume also includes 10 RoboCup Champions Papers. In addition to presenting the proceedings of the RoboCup 2021 Symposium, the book highlights the approaches of champion teams from the competitions. Due to the complex research challenges set by the RoboCup initiative, the RoboCup International Symposium offers a unique perspective for exploring scientific and engineering principles underlying advanced robotic and AI systems.