There had been much speculation about a move for the Italy international, with the player's agent claiming he "would do anything" to move to Milan last month.
However, it was announced on Thursday that the 27-year-old has been loaned to Roma, who finished second in Serie A last season and will play UEFA Champions League football in the upcoming campaign.
Cagliari confirmed the deal on their official Twitter account, and posted a message on their wesbite to express their appreciation for the player.
"Cagliari Calcio thanks Astori for the professionalism, devotion and gratitude shown until the last moment of his time in red and blue," it read.
After moving from Milan in 2008, Astori made 174 Serie A appearances at Stadio Is Arenas, helping the club to a 15th-place finish last season.
He joins Radja Nainggolan in making the switch from Cagliari to the capital, the Belgian midfielder penning a permanent switch at the Stadio Olimpico after spending the latter half of last season on loan with Roma.
