The 28-year-old is set to complete the move by the end of the week.
Riera has failed to make an impact at Anfield since joining the club in a €12million deal from Espanyol two years ago.
Olympiakos manager Ewald Lienen said: “Unless there is a major surprise then he will be our player.
“He is a great player who has proven his worth both at club level and for the Spanish national side.”
