Rabiot was a free agent since leaving Juventus in July after five years in which he won the 2020 Serie A title and two Italian Cups.
"Marseille are delighted to announce the signing of Adrien Rabiot. The midfielder has signed a new contract... after passing his medical," said the Ligue 1 club in a statement.
The length of the former Paris Saint-Germain player's contract has not been specified but a source with knowledge of the matter told AFP on Sunday that he was expected to sign for two years.
Rabiot has played 48 times for France and was a runner-up at the last World Cup in Qatar.
He also played five times at Euro 2024, where France were eliminated in the semi-finals by eventual winners Spain.
On Monday evening, Rabiot, who won six league titles with PSG before a rancourous exit, was welcomed at Marignane airport by several hundred Marseille supporters, who chanted his name and lit flares.
"I wasn't expecting that. Seeing all these people here is heart-warming. I know my family are happy too. It makes you want to play and win," he told the cameras.
"It's really incredible. I didn't expect such a good mood, such warmth," he added.
Marseille are off to a good start in the French top flight this season under new coach Roberto De Zerbi and Saturday's win over Nice was watched by 65,000 at their iconic Velodrome stadium.
De Zerbi brought in the 22-year-old Mason Greenwood in the close season, a controversial signing due to sexual assault charges that were eventually dropped, but the striker has scored five goals in their four league games so far.
Rabiot could feature against Lyon on Sunday, even though he has not played since the Euros.