The Argentine, who sources say was suspended for making a trip to Saudi Arabia last week and missing practice as a result, trained on his own at the Ligue 1 leaders' facility Monday while the rest of the first team was given the day off following a win over Troyes on Sunday.
Sources added that PSG took the decision to shorten Messi's suspension, originally reported by French media to be two weeks, after the World Cup winner issued an apology to the club, his teammates and fans for missing practice last week.
Messi will still be fined two weeks' wages, approximately €1.6 million, the sources added.
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."
Later on Monday, Messi attended the Laureus Awards in Paris, where he was named Sportsman of the Year and also accepted the award for World Team of the Year on behalf of Argentina, who he led to the 2022 World Cup trophy in December.
Messi's future at PSG remains unclear, with a source close to the Argentina captain saying he received a formal offer to join Saudi Arabian club Al-Hilal next season, while adding that a contract renewal at PSG is not in the cards for the 36-year-old.
Saudi Arabia appointed Messi its tourism ambassador last year, and he visited Jeddah in May 2022.
PSG are six points clear of Lens and eight in front of Marseille with four rounds of games remaining this season as they remain on course to defend their Ligue 1 title from last season.
Information from Reuters, The Associated Press was used in this report.