"I spoke with him on Tuesday to see what his mindset was and I found him very determined to play... He will start tomorrow," Galtier told a press conference.
Messi was back in training on Monday after being suspended for making an unauthorised trip to Saudi Arabia and missing a training session as a result.
French media reported that Messi was suspended for two weeks after making the trip when he was supposed to train with the squad the day after the Ligue 1 leaders' 3-1 defeat to Lorient last month.
Messi apologised to PSG and his teammates last Friday and was left out of the squad for their 3-1 win at second-bottom Troyes two days later.
PSG are six points clear of Lens and eight in front of third-placed Olympique de Marseille with four games remaining this season.
Messi's future at PSG has been the subject of much speculation recently, with a source close to the Argentina captain telling Reuters that he had received a formal offer to join Saudi Arabian club Al-Hilal next season.
The player's focus is on winning the French title, however, Galtier said.
"He's very eager to win that title," the coach said.
"With him in the team we'll have to adjust compared to what we did against Troyes. Leo is Leo. I'm not going to go into statistics but when he's with us we have to strike a different balance."
PSG's season has been well below expectations, having suffered early eliminations in the Champions League and the French Cup.
Lens will move provisionally three points behind PSG if they beat Stade de Reims at home later on Friday.
PSG have lost three of their last four home games.
