Eric Abidal joined on Tuesday his new teammates at Zeefeld, Austria for their preseason training after Olympiakos confirmed on Saturday it had signed the veteran French defender on a two-year contract for an undisclosed fee.
Monaco had announced a few days ago that its 34-year-old captain, who underwent a liver transplant in 2012 while playing for Barcelona, would extend his contract with the French Ligue 1 runner-up for another season.
But instead he has joined up with the Olympiakos squad in Austria.
Abidal made 26 appearances in Ligue 1 last season but lost his place to Aymen Abdennour in the second half of the championship and was not called up by France coach Didier Deschamps for the World Cup.
Abidal, who played for Barcelona for six years from 2007, was first diagnosed with a liver tumor in March 2011 and a year later underwent a transplant.
The former Lille and Lyon player says he has fully recovered and he successfully completed medical checks with Olympiakos on Saturday.
He joined Monaco in July last year after Barcelona opted not to renew his contract and won the last of his 67 caps for Les Bleus in February 2012 before undergoing the liver transplant that sidelined him for just under 12 months.
Monaco's Congolese international midfielder Delvin N'Dinga has been loaned for a second season to Olympiakos, the Ligue 1 runners-up confirmed on Tuesday.
The 26-year-old, who has 23 caps for the Congo national side, played 19 matches for the Greek Superleague outfit last season, scoring one goal.
The former Auxerre player teams up again with Abidal.
Another Frenchman, winger Mathieu Dossevi, 26, also joins Olympiakos from Ligue 1 side Valenciennes, the Greek club confirmed.
On Sunday Olympiakos announced the extensio of the contracts of Greece internationals Andreas Samaris and Dimitris Siovas until 2018.
Midfielder Samaris was a member of the Greek squad that reached the last 16 of the World Cup in Brazil before losing to Costa Rica on penalties.
The 25-year-old scored the first goal in a 2-1 victory over Ivory Coast in the group stage, a result that qualified Greece for the knockout phase for the first time in the country's history.
Samaris scored four goals in 28 appearances last season as Olympiakos strolled to a record 41st Greek title.
Siovas, 25, has been capped eight times at international level, but the defender missed the World Cup through injury.