The South Korea defender's departure signals the end of a spell of almost two years with the Hoops.
He arrived in the east end of Glasgow from Bundesliga side Freiburg in July 2010 and now returns to the country of his birth.
A statement on Celtic's official website confirmed: "Cha Du-ri has signed for Fortuna Dusseldorf after spending nigh on two years with the Hoops.
"The German-born South Korean internationalist returns to football in the land of his birth after making 44 appearances for Celtic, during which he scored twice.
"Everyone at the club would like to wish Cha every success in his future career."