The 23 year-old joined the Atalanta first team in 2009 and despite showing great promise he never made a senior appearance for the Orobici.
Cagliari have swooped in to claim the former Italian Under-21 international as revealed through a statement on their official website.
Colombi gave his first interview as a Serie A player having spent much of his career to date playing on loan in Serie B.
“I am very happy to be here,” he told Cagliari’s website “I’d like to thank Cagliari for the trust they’re placing in me.
“I will do my best to repay this trust. I can not wait for the season to start, for me to play Serie A is a dream come true.”
Follow Louis Gibberd-Thomas on Twitter: @mercutio156