This book constitutes the refereed proceedings of the 12th International Symposium on Algorithmic Game Theory, SAGT 2019, held in Athens, Greece, in September/October 2019. The 25 full papers presented together with 3 invited talks and one abstract paper were carefully reviewed and selected from 55 submissions. The papers are organized in topical sections named: Algorithmic Mechanism Design; Auctions and Markets; Computational Aspects of Games; Network Games and Congestion Games; Social Choice; and Matchings and Fair Division.