The Finland international has signed a three-year deal with the Eredivisie champions.
The Amsterdam giants were in the market for defensive reinforcements following the departure of Jan Vertonghen to Tottenham Hotspur, and head coach Frank de Boer has now landed the left-footed defender he set his sights on.
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"Frank de Boer believes that I can become an important player, though, with my specific qualities and my experience."
The 26-year-old Moisander previously played for Ajax from 2003 until 2006, but failed to break into the first team.
Moisander becomes Ajax's fourth signing of the summer transfer window after the previous arrivals of Lasse Schone, Ilan Boccara and Tobias Sana.