The 21-year-old's existing deal was set to expire in 2016, but the new contract sees the playmaker extend his stay at the Amsterdam Arena until June 2018.
Having broken into the first-team picture this season, Klaassen has made 22 appearances in the Eredivisie and contributed a healthy return of nine goals.
And Klaassen is thrilled to be prolonging his tenure with Frank de Boer's champions as they seek a fourth-consecutive crown.
Speaking to the club's official website, he said: "This is a logical step for me. We really want to start building something with Ajax, and I think we are doing well.
"I believe that beautiful things can happen."
Klaassen made his debut for Ajax in November 2011 against Lyon in the UEFA Champions League.
Ajax can secure a fourth Eredivisie title in a row with victory at Vitesse on Sunday, providing Feyenoord fail to beat RKC Waalwijk.
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