The attack-minded player joins the Impact after stints in the top flights of Argentina, France and Italy.
Piatti, 29, becomes Montreal's second current DP, joining Marco Di Vaio. Hernan Bernardello left the club to finalize a move to a Mexican side.
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“To be able to get another DP is another important step for the club," Impact sporting director Nick De Santis said in a statement. "It was a long process but we’re excited to bring him here. “We’ve said since last year that we wanted an offensive midfielder and we are adding today a very good player, at the prime of his career, who was a catalyst for San Lorenzo in their last championship and their run in Champions League. He’s very motivated to play in MLS and to be in Montreal.”
Piatti has played for several teams in the Argentine top flight, including Gimnasia and Independiente. In addition, the 29-year-old has had stints at Saint-Etienne in France's Ligue 1 and Italian Serie A side Lecce.
As San Lorenzo is still involved in the Copa Libertadores, an arrangement has been made that Piatti will join the Impact after the South American club competition, "provided that [Montreal] obtains a special exemption from FIFA to return the player’s registration to MLS outside of the transfer window."