Turkish club Galatasaray has signed winger year contract.
Riera, 29, spent just one year with Olympiacos, but the Spanish international previously had stints with Liverpool, Manchester City, Espanyol, Bordeaux and Mallorca.
"I wish to thank the president and the Olympiacos board for finally accepting my desire to move to Galatasaray as it was an excellent proposal for me and my family," Riera said.
"I believe that I showed the best of me in the year that I was with the team and I wish Olympiacos, my co-players and the fans all success."
Riera has played 16 times for Spain since making his debut in 2007.