Greek international defender Sokratis Papastathopoulos has joined Olympiacos on a deal until 2023.
As reported on GCT, the contract the 32-year-old had at Arsenal was cancelled by mutual consent last week.
After his departure, Sokratis said: "One of the most enjoyable times in my career ended today."
"It has been an honour to wear the Arsenal shirt and I want to thank all the coaches, team-mates, staff and fans around the world for the love and respect they have given me."
Papastathopoulos joined Arsenal from Borussia Dortmund in July 2018 and made 69 appearances for the club.
He was part of the Emirates FA Cup-winning squad in August and played 12 matches during Arsenal’s run to the Europa League final in 2019.
Olympiacos are top of the Greek Super league with 48 points from 18 games, 11 points clear of second-placed AEK Athens.
In a statement posted on Twitter, Papastathopoulos said: "I am happy for this, I have set many goals and I look forward to returning to the field. We have an important season ahead of us and I am sure we will achieve many things in the coming years. Thank you for your support."
Notable players in Olympiakos' ranks include former Bayern Munich right back Rafinha, ex-Rennes and Inter Milan midfielder Yann M'Vila, one-time Watford and Roma left back Jose Holebas and former Marseille midfielder Mathieu Valbuena.
Considered one of Greece's most reliable central defenders, Papastathopoulos has made 90 appearances for his country.