This book constitutes the refereed proceedings of the 9th International Symposium on Algorithmic Game Theory, SAGT 2016, held in Liverpool, UK, in September 2016. The 26 full papers presented together with  2 one-page abstracts were carefully reviewed and selected from 62 submissions. The accepted submissions cover various important aspects of algorithmic game theory such as computational aspects of games, congestion games and networks, matching and voting, auctions and markets, and mechanism design.