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."
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