The Premier League club confirmed the news on their official twitter page.
Denayer has already broken into the Belgium senior squad and took part in City’s summer pre-season tour.
The 20-year-old arrived in Istanbul yesterday and put pen-to-paper on a season long loan with Galatasaray. It is unknown whether Galatasaray have agreed an option to sign the Belgium international.
The Belgium international is understood to be keen to play Champions League football this season.
Denayer scored six goals and setup two assists in 44 appearances for Celtic in all competitions last season.
The 20-year-old who was voted Scotland’s Young Player of the Year last season, turned down ‘a number of offers’ in favour of Galatasaray according to SkySports.
Galatasaray are coming off the back of their first win of the season following a 4-1 victory over Konyaspor.