The 26-year-old, who joins from AZ Alkmaar, has revealed his delight at returning to the club at which he spent two years in 2004.
"It was a hectic summer, but I'm glad it's all come good. Ajax is my dream club. I am very fit and want to make my debut. Hopefully that happens Saturday night in the home match against NAC Breda," he told the club's official website.
Moisander will fill the void left by Tottenham Hotspur defender Jan Vertonghen.