The Impact reached agreement on a contract with Piatti on July 1 and he is to join the Major League Soccer club as a designated player sometime in August.
Sporting director Nick De Santis said it is league policy not to reveal the player's salary or the length of the deal, but that it was a long term contract.
"It's exciting because, first and foremost, he's a very good person who is extremely excited and motivated to come here," De Santis said Wednesday.
"His strengths are his ability to score important goals, his hunger to score goals, his ability to put defences on their heels, his ability to strike the ball with the right foot or the left. He gets in a good attacking positions inside the 18-yard box, and he's a two-way player."
Piatti replaces fellow Argentine Hernan Bernardello, a designated player who left this week to play for Mexican club Cruz Azul. The 29-year-old Piatti will be the team's second DP after striker Marco Di Vaio, who turns 38 on July 15 and who is expected to retire after this season.
A designated player is one who earns more than the league maximum, but only US$387,500 is charged against the team's cap.
The international transfer window opens July 8, but the Impact agreed to allow Piatti to stay with his Argentine club San Lorenzo de Almagro to finish out play in the Copa Libradores, the Latin American Champions League.
San Lorenzo has semifinal matches July 23 and July 30. If they win, the final will be Aug. 6 and Aug. 13. Piatti would need a letter of exemption from FIFA to play the last game because the transfer window closes Aug. 8.
He is to join the Impact after playing his last game. Then he will no doubt need time to adapt to a new country and a new league before he plays his best soccer for Montreal.
When Di Vaio joined the Impact from Italian club Bologna in mid-season in 2012, he was not the scoring machine he became the following season when he had 20 goals.
"We have to understand the adaptation period," said De Santis. "We've seen it with Marco (in 2012).
"It's a long term deal. He's 29. He's a player with a lot of qualities going forward, an offensive midfielder that has the ability to score important goals."
De Santis worked for more than a year on bringing Piatti to Montreal. Reports that the San Lorenzo star's arrival was imminent popped up repeatedly in recent months, but it looked like a deal was close when Piatti visited Saputo Stadium earlier this season.
"A year and a half ago I went to Argentina to see him," said De Santis. "I went again to meet him and his family.
"We tried to bring him here last year, but he couldn't. The whole club did a lot of work. And we have to thank the president (Joey Saputo), who allowed us to get another DP."
Striker Jack McInerney liked the signing.
"I heard he's an attacking mid, so that gets me excited," said the player known as Jack Mac. "Another player that can come in and create chances for myself and Marco and others. It should be interesting."
The five-foot-11 Piatti scored 30 goals in more than 160 Argentine first division matches.
He has played for five clubs in Argentina as well as with St-Etienne in France in 2006 and Lecce in Italy from 2010 to 2012.