Honda and Marquez agreed to terminate their four-year contract a year early, ending an 11-year association and clearing the way for him to switch to a Ducati machine, the dominant force in MotoGP over the last two seasons.
"I'm excited with this new challenge. It wasn't an easy decision because it's a big change in every way," Marquez said in a statement.
"But sometimes in life it's important to get out of the comfort zone and put yourself through the paces in order to keep growing."
The Spaniard has struggled with injury and an uncompetitive bike this season, although he finished a season-high third in a rain-hit and shortened Japanese Grand Prix.
Honda have not been able to match the pace of the Ducati bikes, leading to Marquez attempting to ride his bike beyond its limits and crashing several times.
"Bike-change-wise, I know I will have to adapt my riding style to a few things, and it won’t be easy. But I'm also sure that the whole Team Gresini will help me a lot," he added.
Marquez, who joined the Repsol Honda team in 2013, won the last of his six championships in 2019, finishing 151 points clear of his nearest rival.
He will join his brother Alex, who has impressed since he made the switch from LCR Honda to Gresini ahead of the 2023 season.
"This is a historical moment for the Gresini Family," Gresini boss Nadia Padovani Gresini said.
"The fact that Marc Marquez chose to race with us in the upcoming season is absolutely fantastic and I'm extremely happy to be able to make it official."
