On 11 May 2017, club president Tommaso Giulini announced Cossu would return to Cagliari for the 2017–18 season. He made his third debut for the club, and 247th game overall for Cagliari, in a 2–1 away defeat to Milan. On 8 August 2018 he played his farewell game in a friendly against Atlético Madrid, was ceremoniously substituted at the 7th minute of the game and officially retired as a player.