The 28-year-old Barkley made 100 appearances for Chelsea and scored 12 goals following his switch from Everton in January 2018.
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He will link up with a host of former Premier League players at the Ligue 1 club, including goalkeeper Kasper Schmeichel, who left Leicester City this summer, one-time Arsenal star Aaron Ramsey, as well as Nicolas Pepe, on loan with Nice from the Gunners.
"Midfielder Ross Barkley has joined OGC Nice on a free transfer after four-and-a-half years at Chelsea," the French club said in a statement.
"Though the summer transfer window closed last Thursday evening in France, he can join the Aiglons as he was out of contract."
Barkley joined Chelsea as one of English football's most highly rated players, but never established himself as a regular at Stamford Bridge and spent a season on loan with Aston Villa in 2020-21.
The England international was presented to Nice fans ahead of the Riviera derby against Monaco, which Barkley's new club lost 1-0.
Despite their host of close-season additions, Sunday's defeat left Nice in 16th place with five points from six games. They host German side Cologne in their Europa Conference League group opener on Thursday.
Information from the Associated Press and Reuters was used in this story.