The 28-year-old has spent the last four years in Russia with Rubin Kazan and Spartak, but has seen his chances limited during the current campaign.
With Milan having only kept four clean sheets in all competitions under Filippo Inzaghi they were quick to snap up the services of Bocchetti, who previously played for Genoa in Serie A.
Inzaghi will hope the Italy international can help turn Milan's fortunes around after three successive defeats, leaving them out of the Coppa Italia and 11th in the league.
Bocchetti will have to hit the ground running despite not playing a competitive match since November, as Milan look to ease the pressure on Inzaghi and attempt to climb up the table.
The defender's debut could come in this weekend's clash with Parma at San Siro given there will be a space at the back following Philippe Mexes' four-game ban for a red card against Lazio.
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