The Iceland striker, who joined Ajax in 2011 from AZ, has put pen to paper on a one-year extension with the Amsterdam club.
Sigthorsson had verbally agreed to further his stay at the Amsterdam Arena back in August, and now the formalities of the deal have been completed.
However, the 24-year-old will not have the chance to immediately mark the agreement with a goal, due to the ankle injury he suffered in last Saturday's 5-0 victory over Willem II.
That blow will keep Sigthorsson out until the new year, placing more emphasis on top-scorer Arkadiusz Milik – who has 16 goals to his name in all competitions – to deliver the goods in Ajax's final two league games before the mid-season break.
Ajax host APOEL in their final UEFA Champions League group match on Wednesday, before welcoming Utrecht to the Dutch capital on Sunday.
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