The Italian only took charge of the Istanbul club in October but has departed despite finishing runners-up in the Super Lig and winning the Turkish Cup last season
The 49-year-old former Manchester City manager was appointed Fatih Terim's successor back in October and signed a three-year-deal upon his arrival in Istanbul.
But, while Mancini lifted the Turkish Cup in his first season in charge, he was unable to stop rivals Fenerbahce from winning the Super Lig title, while Gala exited the Champions League at the hands of Chelsea in the round of 16.
A statement on the club's official website read: "By Mr Mancini's request and by mutual agreement, the contract was terminated.
"According to the agreement, Mr Mancini does not have any further external demands and the parties have agreed to part ways with full friendship and understanding.
"We want to thank him for his efforts and wish him success and good luck for the future."
The news comes as a surprise after Mancini reaffirmed his commitment to the Galatasaray cause just two days ago, insisting he would see out the remainder of his contract.
He had previously been linked with a return to Italy with Lazio, the club where he spent four years as a player between 1997 and 2001.