The seven-time Ballon d'Or winner was initially given a two-week club suspension for travelling to Saudi Arabia for commercial reasons without PSG's knowledge or permission.
Messi - who is a tourism ambassador for Saudi Arabia - jetted off to the Middle East the day after PSG's 3-1 home defeat to Lorient, having erroneously believed that the team would have been given a day off on Monday as per usual.
However, Galtier called his players to their Camp des Loges training base to report for training as normal on Monday, by which time Messi was already on his way to Saudi Arabia.
The Argentine World Cup winner was subsequently fined an unspecified amount and exiled from the first team, missing last weekend's 3-1 triumph at relegation-threatened Troyes.
Last week, Messi posted a public apology to the club and his teammates on social media, claiming that he could not postpone his commercial commitments again and would wait to see what further action, if any, PSG would take.
The powers-that-be were satisfied with Messi's remorse, and he was subsequently welcomed back to individual training on Monday while the rest of the team enjoyed a post-victory rest day.
The attacker's return to training did not necessarily guarantee that he would be back in the first-team picture this weekend, but Galtier has confirmed that Messi will start in Saturday's clash.
"I spoke to Lionel Messi about his return on Tuesday to see what kind of mindset he was in. He was very calm, and I thought that he was very motivated to play and determined to win another trophy. Yes, he will start tomorrow," Galtier said at Friday's press conference.
After posting a mere six goals in his debut campaign in Ligue 1 last season, having struggled to adapt to life outside Barcelona, Messi has hit his stride this term with 20 goals and 19 assists from 37 outings in all tournaments.
The 35-year-old has contributed 15 goals and 15 assists in 28 Ligue 1 contests for PSG, although he is seemingly set to leave the French champions when his contract expires at the end of the season.
With Messi back in the fold, Hugo Ekitike could drop out of the first XI - although Galtier has hinted at a possible front three for Saturday's match - while key defender Achraf Hakimi is also back from a one-game ban.
Galtier is still coping without several long-term absentees, though, as Neymar (ankle), Presnel Kimpembe (Achilles), Nuno Mendes and Nordi Mukiele (both hamstring) sit out the remainder of the season.
Up-and-coming defender Timothee Pembele is also absent once again due to a knee problem, as PSG aim to increase or maintain their six-point lead over Lens at the top of the Ligue 1 table.
For 18th-placed Ajaccio, who sit 11 points adrift of safety with four games remaining, anything other than a shock win at the Parc des Princes would confirm their demotion back down to Ligue 2.
PSG SQUAD:
Donnarumma, Rico, Mouquet; Pereira, Marquinhos, Ramos, Bernat, Hakimi, Bitshiabu, Nhaga; Vitinha, Verratti, Ruiz, Soler, Sanches, Gharbi, Zaire-Emery; Messi, Mbappe, Ekitike, Housni