French media reported that Messi was suspended for two weeks after an unauthorised trip to Saudi Arabia when he was supposed to train with the squad the day after the Ligue 1 leaders' 3-1 defeat to Lorient last month.
Messi, who apologised to PSG and his teammates on Friday, was left out of the squad for their 3-1 win at second-bottom Troyes on Sunday.
PSG posted a picture of Messi training on social media, along with the caption: "The Argentinian striker from Paris Saint-Germain was back in training on Monday 8 May."
PSG are six points clear of Lens and eight in front of Olympique de Marseille with four more rounds of games remaining this season.
Messi's future at PSG has been the subject of much speculation in recent days.
A source close to Argentina's captain told Reuters that he received a formal offer to join Saudi Arabian club Al-Hilal next season, with a contract renewal at PSG not on the cards for the World Cup winner who turns 36 next month.
The oil-rich nation appointed Messi its tourism ambassador last year and he visited Jeddah in May 2022.
He returned in January to play a friendly match with PSG against a team of Saudi League stars when he faced great rival Cristiano Ronaldo.
