The Netherlands goalkeeper's previous deal was set to run out in June 2016, but the 25-year-old has agreed fresh terms until 2018.
And Cillessen is thrilled to be extending his stay at the Amsterdam Arena.
"Once again, this is a contract that exudes trust," he told the club's official website. "A contract extension with Ajax remains special.
"18 months ago, everything was different. That shows how fast things can move. A lot has happened in the last year."
Cillessen moved to Ajax from NEC in August 2011 for a reported €3 million fee and he has gone on to make 69 appearances for the first team.
Ajax coach Frank de Boer believes the new deal is a good sign for the ambitions of the club.
"This sends a good signal," he said. "Jasper, along with Davy Klaassen, Joel Veltman and Ricardo van Rhijn, among others, have bound themselves to the club for a longer period of time.
"If the current group remains together for a minimum of two years great things will happen."
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