Olympiacos B.C. (Greek: ΚΑΕ Ολυμπιακός Σ.Φ.Π.), commonly known as Olympiacos or 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, and a basketball department was first established in 1931. Their home ground is the Peace and Friendship Stadium.

Olympiacos has been established as one of the most successful clubs in Greek and European basketball, having won 3 EuroLeagues, 1 FIBA Intercontinental Cup, 15 Greek Championships, 12 Greek Cups, and 4 Greek Super Cups. Among Greek clubs, Olympiacos is the first ever to reach a EuroLeague final, as well as to achieve a Triple Crown, and the only one to be crowned back-to-back European champions. Olympiacos is also the Greek club with the most appearances in EuroLeague finals, 9 in total.

The first major achievement of Olympiacos in European competitions was their qualification for the 1978–79 FIBA European Champions Cup semi-finals group stage. However, it was in the 1990s that Olympiacos emerged as Euroleague contenders, having been runners-up in two consecutive seasons (1994, 1995) and winning their first title in 1997 by beating FC Barcelona 73–58, which was at the time the competition's biggest winning margin in a single-game final. As European champions, they participated in the 1997 McDonald's Championship, where they played against the NBA champions, Michael Jordan's Chicago Bulls. During this period, Olympiacos also dominated the Greek Championship, which had become one of Europe's top national basketball leagues, winning five consecutive titles. Thus, FIBA declared Olympiacos as the best European team of the 1990s.

Olympiacos reached again the EuroLeague final in 2010 and returned to the top of European basketball in 2012, when they won their second EuroLeague title by overcoming CSKA Moscow 62–61 on the last shot of the game, rallying from 19 points down, and achieving the greatest comeback in the competition's finals history. In 2013, Olympiacos won their third title after defeating Real Madrid 100–88, pulling off another memorable comeback, this time overturning a 17-point deficit. These consecutive triumphs, along with two more final appearances (2015, 2017), made the 2010s the most successful decade in the club's EuroLeague history. However, on the domestic front, growing tensions regarding refereeing decisions during the 2018–19 Greek Basket League season resulted in Olympiacos refusing to play against their archrivals, Panathinaikos Athens, and consequently the club was relegated to the Greek A2 Basket League.

Olympiacos returned to the top-tier Greek League after a two-year absence, during which they continued to compete in the EuroLeague. In recent years, Olympiacos have once again become one of the most consistent contenders for the European crown, having been runners-up in 2023, as well as having qualified for four consecutive Final Fours, which is a new record for the club.

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, Artūras Karnišovas, Dino Radja, Thodoris Papaloukas, Alphonso Ford, Tyus Edney, Arvydas Macijauskas, Miloš Teodosić, Nikola Vujčić, Linas Kleiza, Rašho Nesterović, Kostas Papanikolaou, Kyle Hines, Acie Law, Georgios Printezis, and Vassilis Spanoulis. In 2008, Olympiacos signed NBA player Josh Childress, whose US$20 million net income contract for three years made him the highest-paid basketball player of all time in Europe.

Olympiacos basketball club has its origins in the 1930s, when the first attempts to establish it were made, in 1931 and then again in 1937. After World War II, the "red-and-whites" were the first Greek team to familiarize themselves with American basketball, as Alekos Spanoudakis learned to use the jump shot, and his brother, Ioannis Spanoudakis (who was both a player and the head coach of the team), met basketball legend Bob Cousy and practiced many of his techniques on the court. During the Spanoudakis brothers' era, Olympiacos won their first Greek Championship in 1949, as well as a second in 1960, and were consistently among the best Greek teams throughout the 1950s. As Greek champions, they also qualified for their first European competition, the 1960–61 FIBA European Champions Cup.

However, this last success was followed by a period of sharp decline, and in 1964, as the Spanoudakis brothers' playing days were coming to an end, Olympiacos was relegated to the Greek Second Division. In early 1967, Faidon Matthaiou, the so-called "patriarch" of Greek basketball, took over as the head coach, and, under his guidance, the club was promoted back to the First Division. In the late 1960s and early 1970s, the Reds were a decent and tough team, with key players such as Thanasis Rammos, Tolis Spanos, Makis Katsafados, Manolis Efstratiou, and Stelios Amerikanos, but they were not yet capable of claiming the championship against the dominant forces of Athenian and Greek basketball at that time, AEK and Panathinaikos.