The Argentine had been suspended without pay for two weeks after an unsanctioned trip to Saudi Arabia last week.
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Messi jetted off to the Gulf state on Monday as part of his duties as Saudi Arabia's tourism ambassador, something that angered PSG chiefs as the rest of the team trained.
It led to the World Cup winner being punished by the club but they have since reversed that decision with immediate effect.
PSG boss Christophe Galtier said in his press conference on Friday: "I spoke with him [Messi] on Thursday.
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"He is motivated, determined and he wants to win another trophy. Yes, he will be a starter tomorrow."
The French champions take on Ajaccio at the Parc des Princes as they look to close in on the Ligue 1 title.
They are six points clear of second-placed Lens with four games left and are the favourites to go on and snap up the trophy.
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As for Messi, he missed one match for PSG when they beat Troyes 3-1 last weekend.
Videos circulated on social media of PSG ultras venting their fury by singing anti-Messi chants outside the ground.
The Argentina international issued a grovelling apology last week for his actions but insisted that he believed he flew to Saudi Arabia on a rest day for the squad.
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With the 35-year-old's contract coming to an end this summer, it has fuelled speculation that he is ready to depart PSG and leave for the Middle East.
There are reports that the former Barcelona star has a contract on the table worth £320million-a-year, nearly double as much as what Cristiano Ronaldo is on at Al Nassr.
And there are even suggestions that Messi could play with his old rival in Riyadh, though Al Hilal are said to be the favourites.
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Other possible destinations include a return to Barcelona and a big-money move to the MLS where David Beckham's Inter Miami are keen on a transfer.
PSG had been confident that the forward wanted to stay in Paris earlier this year after winning the World Cup in Qatar.
But crashing out of the Champions League at the first round, for the second year in a row, has knocked his confidence in the team's ability to go on and win the tournament in the future.