"During the last two years at Liverpool, I didn't win any titles, so that's why I took the decision to come to Olympiakos; to win titles," said the 28-year-old at a news conference.
"I have come to Olympiakos at the right time," added Riera, whose brother, Sito, also plays in Greece for Panionios.
"I hope to help the team become one of the 10 best sides in Europe."
Local media reported the deal would cost 20 million euros, with the player earning around 2.7 million euros per season and Liverpool receiving 4 million euros up front and an additional 2 million if Olympiakos win the title this season.
The Spaniard joined Liverpool for 12 million euros two years ago from Espanyol. After a promising start at the club he faded into the first team fringes and was frozen out after criticising previous manager Rafa Benitez.
After winning nine titles in 10 years, Olympiakos only managed to qualify for the Europa League last season. The Reds beat Besa Kavaje of Albania 6-1 in the second leg of their second qualifying round tie on Thursday, as Riera watched from the stands.
The win secured an 11-1 win on aggregate and a tie against Maccabi Tel Aviv of Israel in the next round.
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