Olympiacos B.C., commonly referred to as Olympiacos and Olympiacos Piraeus, is a Greek professional basketball club based in Piraeus, part of the major multi-sport club, Olympiacos CFP. The parent club was founded in 1925, with the basketball team created in 1931, and their home ground is the Peace and Friendship Stadium in Piraeus.

Olympiacos has been established as one of the most successful clubs in Greek basketball history and one of the most successful clubs in European basketball, having won three EuroLeagues, one FIBA Intercontinental Cup, one Triple Crown, thirteen Greek League titles and ten Greek Cups. As a traditional European powerhouse, Olympiacos have also been five times EuroLeague runners-up and, having played a total of eight finals, they are the Greek club with the most EuroLeague Final appearances. They have also participated in ten EuroLeague Final Fours.

The first major achievement of Olympiacos in European competitions was their presence in the 1978–79 FIBA European Champions Cup semi-finals group stage, but it was in the 1990s that Olympiacos made their biggest mark. They became the first ever Greek club that reached the EuroLeague Final, being runners-up in two consecutive seasons (1994 and 1995), winning their first EuroLeague title in 1997, after a convincing 73–58 win to Barcelona in Rome, which was a record margin win at the time for one-leg final in the competition's history. In the same season, they achieved the first Triple Crown for a Greek team and as European champions they played in the 1997 McDonald's Championship, where they met in the final the Michael Jordan's NBA champions, the Chicago Bulls. During the 1990s, besides their constant achievements in EuroLeague, also adding a third place in 1999, Olympiacos dominated the Greek Basket League with five consecutive titles, at a time when the Greek championship was considered Europe's best national basketball league. Thus, FIBA declared Olympiacos as the "Best European Team of the 1990s".

Olympiacos returned to the very top of European basketball in 2010, when they reached the final against Barcelona in Paris, but mostly in 2012, when they won their second EuroLeague title in Istanbul, by rallying from 19 points down in the championship game, to beat CSKA Moscow 62–61, on the last shot of the game, achieving the greatest comeback in European basketball finals history, and one of the greatest ever seen in European continental basketball. In 2013, Olympiacos won their third EuroLeague title and became the only Greek club and only the third club in European basketball history to be crowned back-to-back European champions in the modern EuroLeague Final Four era, after beating Real Madrid 100–88 in the London final. Later on, Olympiacos won the Intercontinental Cup, celebrating a third international title in 17 months.

Some of the greatest players in European basketball have played for Olympiacos over the years including: Charlie Yelverton, Carey Scurry, Žarko Paspalj, Giorgos Sigalas, Dragan Tarlać, Walter Berry, Panagiotis Fasoulas, Roy Tarpley, Eddie Johnson, Alexander Volkov, David Rivers, Chris Welp, Artūras Karnišovas, Arijan Komazec, Dino Rađja, Theo Papaloukas, Alphonso Ford, Tyus Edney, Arvydas Macijauskas, Ioannis Bourousis, Miloš Teodosić, Nikola Vujčić, Josh Childress, Linas Kleiza, Rašho Nesterović, Kostas Papanikolaou, Kostas Sloukas, Kyle Hines, Joey Dorsey, Stratos Perperoglou, Acie Law, Georgios Printezis and Vassilis Spanoulis. Under the ownership of billionaire Greek brothers Panagiotis Angelopoulos and Giorgos Angelopoulos, Olympiacos made a record transfer in 2008, by signing NBA player Josh Childress, whose US$20 million net income contract for three years made him the highest-paid basketball player in the world ever, outside the NBA.