The MLS club have announced that the former Italy international has signed a deal and will wear the No.14, but the veteran will not be classified as a Designated Player
Nesta, 36, joins the Impact after a glittering Serie A career with Lazio and AC Milan and he can't wait to start his MLS adventure in Montreal.
"I'm very happy to be here," he said at a press conference. "Despite some offers to continue my career in Europe, I was looking for a new challenge and wanted to play in MLS.
"Upon visiting my friend Marco [Di Vaio] to see him play his first game with Montreal, I met great people in this club. I really liked what I saw and I was convinced Montreal was where I wanted to be."
Impact sporting director Nick De Santis was delighted with the transfer coup and heaped praise on the World Cup winner.
"It's exciting not only for the team, but also for the club to have a player of his pedigree, a world champion in 2006 and one of the top defenders in the world," he stated.
"Alessandro will bring a lot of leadership qualities through his different experiences, and will be a great mentor to our young players. To have him around the team and on the field will help the club as a whole."
Impact head coach Jesse Marsch can't wait to unleash his new signing on the opposition and believes the team will improve drastically with the Italian in the side.
"It's an incredible opportunity for our club to add a defender of his talent. We are very anxious to get him into our line-up because he will make us better on every level," he said.
The Impact traded allocation money to the New York Red Bulls in exchange for first priority in the MLS allocation order. Nesta's contract with Montreal won't take up a designated player slot.