It was announced on Sunday that the Italian had penned a three-year deal at San Siro on his 16th birthday, and he will remain at the club until at least June 2017.
Mastour has yet to make his first-team debut for Milan, but he was an unused substitute for the 2-1 victory over Sassuolo on May 18.
A statement on Milan's official website read: "A pleasant surprise for the 16th birthday of Hachim Mastour.
"The young talent today signed his first professional contract which will keep him at Milan until June 30, 2017.
"Hachim Mastour, best wishes from all the Rossoneri fans."
Milan endured a difficult campaign in Italy's top flight last season, eventually posting an eighth-placed finish.
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