Olivier Ntcham left Celtic at the end of his contract after a four-year stint with the Scottish Premiership club.
The French midfielder, who joined Manchester City in 2017, has won three straight league titles at Celtic Park, making 150 appearances for the club.
Ntcham spent the second half of last season on loan at Marseille, but joined Celtic’s pre-season training camp last month under new manager Ange Postecoglou.
In an Instagram post, accompanied by a photo of a late winner at Lazio in the Europa League in November 2019, Ntcham wrote: “I want to thank everyone at Celtic for the 3 years in two (sic) I was there.
“Thank you to all the coaches, staff, players and everyone who works at the club.
“Finally, I would like to thank the loyal supporters who have always supported us.
“I wish you all the best for the future.”
Ntcham initially expressed his desire to leave Celtic in the summer of 2019, but after a potential move to Porto failed to materialize he was reinstated in the squad under Neil Lennon.
A statement from the Celtic club added: “The club can confirm that Olivier Ntcham’s contract with #CelticFC has come to an end.
“Everyone at the club wishes Oli nothing but success for the future.”
André Villas-Boas, then manager in Marseille, offered his resignation following the signing of Ntcham – citing a disagreement over the sports policy of the Ligue 1 club following the transfer.
Marseille did not take their option to buy Ntcham, who has only made two starts in France, but he was at their friendly against Villarreal on Saturday while Celtic lost 2-1 to Hearts in their home game. Scottish Premiership opens at Tynecastle.