The PSG boss revealed in his press conference on Thursday that Messi will play his last game this weekend against Clermont Foot.
He said: "I had a privilege of coaching the best player in the history of football.
"It will be Leo's last match at the Parc des Princes against Clermont."
There had been mounting speculation that Messi would indeed leave PSG when his contract expires on June 30.
It has now been confirmed by Galtier, on what could be his last game as manager amid growing rumours he could be sacked.
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Messi penned a whopping £500,000-a-week contract in 2021 for two years when his terms at Barcelona expired.
Despite attempts from the club to try to get the seven-time Ballon d'Or winner to stay, the decision has been made to leave the Parc des Princes.
A romantic return to Barcelona appears unlikely, though, with the finances surrounding any deal almost impossible to pull off from the Catalan club.
Instead, a move to Saudi Arabia looks most probable where Al Hilal could snap up the World Cup winner.
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It would see Messi and Cristiano Ronaldo reignite their rivalry with the Portuguese icon currently with Al Nassr.
But it might be an acrimonious exit from PSG for the Argentine this weekend after being barraged by boos from his own fans in recent weeks.
An unsanctioned trip to Saudi Arabia, where he is the nation's tourism ambassador, when PSG were training earned him a two-week suspension.
After another season where the French giants were knocked out of the Champions League round of 16, tensions have already been heightened.
And PSG fans have voiced their displeasure at Messi, as well as at Neymar and the manager.
Galtier added: "He was an important part of the team this season, and was always available.
"I don't find the remarks and criticism to be justified."
Despite the criticism, Messi has still contributed 21 goals and 20 assists in all competitions this year.
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His last goal also helped PSG win the title this season, scoring in a 1-1 draw against Strasbourg that confirmed their Ligue 1 victory.
Whether it will be his final campaign in Europe remains to be seen while the 35-year-old will be desperate to win the Champions League at least one more time.