The 31-year-old has agreed a three-year deal after Sevilla accepted an offer of up to €4.5 million for the right-back.
Mariano, who started 29 La Liga games for Sevilla last season, will earn €2.3m per season during his time in the Super Lig.
"I didn't give a second thought when Galatasaray came calling," Mariano said. "This is a well known club in Europe. I spoke to [former Gala goalkeeper] Claudio Taffarel about the club and Istanbul."
Hoş geldin Mariano! pic.twitter.com/WDdhLw4YRh— Galatasaray SK (@GalatasaraySK) July 17, 2017
The club also allowed Netherlands international Wesley Sneijder to leave despite the midfielder having a year remaining on his contract.