The 2007 Ballon d'Or winner triggered a non-Champions League release clause in his San Siro contract on Monday, having rejoined Milan in September 2013, four years after quitting for Real Madrid for a fee of £58m.
Reports immediately suggested that Kaka was on his way to Orlando, who have been promoted to an enhanced MLS for next season, and the 32-year-old Brazilian has confirmed this latest step in his career, via a loan spell at Sao Paulo until January 2015.
"I've always said I wanted to play in the USA and now I have reached an agreement to play there," Kaka told reporters.
"I'm happy to return to Sao Paulo, it's really satisfying for me," he added. "Orlando City is a new team with a good project and the owner [Flavio Augusto da Silva] is Brazilian."
Kaka, who won Serie A titles and a Champions League during his first spell at Milan between 2003 and 2009, made a total of 307 appearances for the Rossoneri, scoring 104 goals.