Di Vaio moves to Canada after leading Bologna, where he scored 65 Serie A goals in 143 games over four years, to Serie A safety last season.
"I am very proud to become a member of the Montreal Impact and to start this new phase of my career," Di Vaio said on the club's website.
"I have accepted with great enthusiasm the challenge of joining an enticing championship like MLS. I am extremely eager to make the entire club proud, from the president to its supporters."
The 35-year-old becomes Montreal's first signing under the Designated Player Rule which allows MLS teams to buy one player outside their salary cap, allowing them to bid for high-profile international players.
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