Ziyech's temporary contract with Galatasaray includes an obligation to make the move permanent at the end of the season.
Ziyech had been exploring moves to leave Chelsea since the start of this year, with a loan move to Paris Saint Germain in January falling through due to administrative issues and another switch to Saudi Arabian side Al Nassr not materialising earlier this summer because of a knee problem detected during his medical.
The 30-year-old joined Chelsea from Ajax Amsterdam in the summer of 2020 and made 107 appearances for the Stamford Bridge club, scoring 14 goals and assisting 13 times.
He won the UEFA Champions League, UEFA Super Cup and FIFA Club World Cup during his time at Chelsea.
Galatasaray have also secured permanent deals for Wilfried Zaha and Mauro Icardi this summer.
Ziyech starred for Morocco at last year's FIFA World Cup in Qatar as his side became the first African team to reach the semi-finals.
