The 35-year-old's 18-year career at San Siro came to an end last month when it was decided not to extend the veteran enforcer's contract, and La Viola were quickest to move for the Marchese despite reported interest from Premier League club West Ham.
A statement published on Viola Channel read: "ACF Fiorentina have completed the signing of midfielder Massimo Ambrosini on a permanent contract - the former Italy midfielder putting pen to paper on a one-year deal until June 30, 2014."
Ambrosini won four Serie A titles, two Champions League trophies, one European Super Cup, one Club World Cup, one Coppa Italia and two Italian Super Cups during his time in Milan. He also made 35 appearances for Italy in a nine-year period but was not included in the 2006 World Cup-winning squad.
His arrival at the Stadio Artemio Franchi bolsters an already strong midfield that includes Alberto Aquilani, Borja Valero, David Pizarro and winger Joaquin as head coach Vincenzo Montella looks to build on a fourth-placed finish in the 2012-13 campaign.