The Spanish winger was presented to media today after fans lined the streets to welcome his arrival at the Athens club.
The 28-year-old fell out of favour at Anfield last season after an outspoken attack on former manager Rafael Benitez and describing the Reds as a "sinking ship".
Reports claim the deal is worth £3.3million and that Riera has signed a four-year contract.
Riera joined Liverpool from Espanyol two years ago but failed to impress regularly, scoring just five goals in 56 appearances.
He was linked with Spartak Moscow last season but his wage demands were thought to have priced him out of a move.
Speaking about his motivation for the move, Riera said: "I am eager for titles and honours.
"I am comfortable with my decision and I will be playing with a group hunting for trophies.
"I came to win titles, which are missing from my career."
Riera admits he was taken aback by the reception he received from fans, who turned out in huge numbers to greet him.
He added: "I was told the fans were coming but what I saw was much more than I expected.
"I did not expect so many people, honestly. It was a surprise for me.
"Such enthusiasm is good for the club. I understand how important the support of the people is, it's priceless."
Riera joins his younger brother Sito, of Panionios, in playing in the Greek league.