The 29-year-old moves to San Siro on a three-year contract and his arrival comes just a day after Christian Abbiati agreed a one-year extension with the club.
Agazzi had been at Cagliari since 2010, but spent the latter half of this season at Chievo, having been frozen out due to his refusal to sign a new deal at the Stadio Is Arenas.
"AC Milan wishes to communicate the signing of Michael Agazzi on a contract which runs to 30 June 2017," read a brief statement on Milan's website.
Agazzi will likely serve as back-up to Abbiati, with current deputy Marco Amelia set to leave after four years with the club.
Amelia's agent, Francesco Zavaglia, confirmed recently that his client was "very unlikely" to renew his contract.
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