This book constitutes the refereed proceedings of the 8th International Symposium on Algorithmic Game Theory, SAGT 2015, held in Saarbrücken, Germany, in September 2015. The 22 full papers presented together with one extended abstract and 6 brief announcements were carefully reviewed and selected from 63 submissions. They cover various important aspects of algorithmic game theory, such as matching under preferences; cost sharing; mechanism design and social choice; auctions; networking; routing and fairness; and equilibrium computation.