The Gialloblu had Birsa on loan at the Stadio Marcantonio Bentegodi last season and they have decided to buy him outright from the Rossoneri, agreeing a three-year contract.
A report on the club’s official website announced the news, saying that the 28-year-old is now tied to the club until June 30 2018.
The Slovenian midfielder appeared 36 times for Chievo while on loan at the club last season, providing one assist.
He joined Milan in 2013 from Genoa, appearing 21 times for them with two goals before spending the next season with the Flying Donkeys.
Birsa has also had spells in France with Auxerre and Sochaux before moving to Genoa in 2011.