The 20-year-old spent 2013-14 on loan at Augsburg, scoring twice in 18 Bundesliga appearances.
After securing their fourth straight Eredivisie title last month, Ajax opted against renewing their loan deal with Barcelona for forward Bojan Krkic.
The club's director of football Marc Overmars is looking forward to seeing Milik in action at the Amsterdam Arena, and expects the Poland international to provide the Dutch champions with a different type of attacking threat.
"At Ajax we wanted to attract a target man for next season," he told Ajax's official website. "I think Milik has this profile.
"He is a player with the ball can keep with him because he is physically strong, he has a good shot and is dangerous in the penalty area."
Ajax have the option to buy Milik should his season-long loan go well, and the player is relishing the chance to test himself at one of Europe's leading clubs.
"Ajax is a big club with a good coach," he said.
"I hope to be in the team next season.
"I look forward to playing in the Dutch league, and of course I hope to be able to score many goals."
