Mariano said goodbye to his teammates and the technical team after his last training session, Istanbul side said on Twitter.
"Thanks for your efforts. Good luck, Mariano!" Galatasaray added.
The 34-year-old joined the club from Spain's Sevilla in 2017 and has scored two goals and 16 assists in 105 games.
Mariona helped the Lions win two Turkish Super Lig titles, one Turkish Cup trophy and one Turkish Super Cup title.
He also clinched Europa League trophy with Sevilla in 2016.
