Olympiacos Football Club (Greek: ΠΑΕ Ολυμπιακός Σ.Φ.Π. Greek pronunciation: [olimbiaˈkos]), also known simply as Olympiacos, Olympiacos Piraeus or with its full name as Olympiacos C.F.P. (Oλυμπιακός Σύνδεσμος Φιλάθλων Πειραιώς Olympiakós Sýndesmos Filáthlon Peiraiós, "Olympic Club of Fans of Piraeus"), is a Greek professional football club based in Piraeus, Athens. Part of the major multi-sport club Olympiacos CFP, their name was inspired from the ancient Olympic Games and along with the club's emblem, the laurel-crowned Olympic athlete, symbolize the Olympic ideals of ancient Greece. Their home ground is the Karaiskakis Stadium, a 32,115-capacity stadium in Piraeus.

Founded on 10 March 1925, Olympiacos is the most successful club in Greek football history, having won 44 League titles, 27 Cups (17 Doubles) and 4 Super Cups, all records. Τotalling 75 national trophies, Olympiacos is 9th in the world in total titles won by a football club. The club's dominating success can be further evidenced by the fact that all other Greek clubs have won a combined total of 39 League titles, while Olympiacos also holds the record for the most consecutive Greek League titles won, with seven in a row in two occasions (1997–2003 and 2011–2017), breaking their own previous record of six consecutive wins in the 1950s (1954–1959), when Olympiacos was unequivocally nicknamed Thrylos (Greek: Θρύλος, "The Legend").

In European competitions, Olympiacos best performances are their presence in the UEFA Champions League quarter-finals in 1998–99, losing the semi-final spot in the last minutes of their second leg match against Juventus, as well as in the UEFA Cup Winners' Cup quarter-finals in 1992–93. The Red-Whites are by far the highest ranked Greek club in the UEFA rankings, occupying the 35th place in Europe in the five-year ranking and the 31st in the ten-year ranking as of 2019, and one of the founding members of the European Club Association. Olympiacos won the Balkans Cup in 1963, at a time when the competition was considered the second most important in the region after the European Cup, becoming the first ever Greek club to win an international competition.