Panathinaikos Football Club (Greek: ΠΑΕ Παναθηναϊκός Α.Ο. [panaθinaiˈkos] ^ⓘ), known as Panathinaikos, or by its full name, and the name of its parent sports club, Panathinaikos A.O. or PAO (Παναθηναϊκός Αθλητικός Όμιλος; Panathinaïkós Athlitikós Ómilos, lit. 'Pan-Athenian Athletic Club'), is a Greek professional football club based in Athens, Greece.

Panathinaikos was founded in 1908 as "Podosfairikos Omilos Athinon" (Football Club of Athens) by Giorgos Kalafatis. They play in Super League Greece, being one of the most successful clubs in Greek football and one of the three clubs which have never been relegated from the top division. Among their major titles are 20 Greek Championships, 20 Greek Cups, achieving eight times the Double and 3 Greek Super Cups. They are also one of three clubs to win a Greek championship undefeated, going without a loss in a top-flight campaign in the 1963–64 season. Panathinaikos is the only Greek team that has reached the UEFA Champions League final in 1971 (which they lost to Ajax 2–0), and also the semi-finals twice, in 1985 and 1996. It is also the only Greek team that has played for the Intercontinental Cup (1971). Furthermore, they have reached the quarter-finals of the Champions League on another two occasions (in 1992 and 2002), as well as the quarter-finals of the UEFA Cup twice (1988 and 2003). They have also won the Balkans Cup in 1977. According to research and polls, Panathinaikos is the second most popular football team in Greece. Panathinaikos is also a member of the European Club Association.

They have played their home games at the Leoforos Alexandras Stadium, considered their traditional home ground, and the Athens Olympic Stadium. Panathinaikos hold a long-term rivalry with Olympiacos, the clash between the two teams being referred to as the "Derby of the Eternal Enemies".

According to the official history of the club, Panathinaikos was founded by the 17 years old track and field athlete Giorgos Kalafatis on 3 February 1908, when he decided to break away from Panellinios Gymnastikos Syllogos following the club's decision to discontinue its football team. The young athlete was followed by the athletes Alexandros Kalafatis (the founder's brother), and Mr. Doukakis, Bouboulis, Chrisis, Granitsas, Mantzakos, Papageorgiou, Gaetas, Demertzis, Stavropoulos, Paschos, Misakian, Reppas, Sapounias and Garoufalias.

The name of the new club was "Podosferikos Omilos Athinon" (Football Club of Athens). It was founded with the aim of spreading and making more known this new sport (football) to the Athenian and Greek public in general. Also, the intention of the founders was to create a team for all of Athens and to be connected with the rest of the European football movement, which was already active. According to Loukas Panourgias, "they wanted their Club not to be like all the others, but a team for Athens, for the entire capital..." Establishing a football-only club at that time was a challenge to the prevailing norms of Greek society."The founding of a football club at a time when this sport was universally regarded as a street game and was openly persecuted by everyone, both authorities and non-authorities alike, was a clear act of rebellion against the prevailing perceptions of sports at the time(Athlitismos, 1927)."

The first president elected was Alexandros Kalafatis, brother of Giorgos. The ground of the team was in Patission Street. Oxford University athlete John Cyril Campbell was brought in as coach, the first time that a foreigner was appointed as the coach of a Greek team. Konstantinos Tsiklitiras, the great Greek athlete of the early 20th century, played as goalkeeper for the new team.

During the turbulent 1910s, marked by the Balkan Wars and World War I, football activity declined. Goalkeeper Konstantinos Tsiklitiras volunteered for the army, fought at the Battle of Bizani, contracted meningitis, and died in Athens at 24. Founder Giorgos Kalafatis served in all three wars, eventually reaching the rank of rear admiral and later on served a military doctor in the Asia minor campaign. In 1910, after a dispute among a number of board members, Kalafatis with most of the players—also followed by Campbell—decided to pull out of POA and secured a new ground in Amerikis Square and the next year the team won the 1911 SEGAS Championship. Subsequently, the name of the club changed to Panellinios Podosferikos Omilos ("Panhellenic Football Club") and its colours to green and white. By 1914, Campbell had returned to England but the club was already at the top of Greek football with players such as Michalis Papazoglou, Michalis Rokkos and Loukas Panourgias and went on to win the football tournament of 1915.

In 1918, the team adopted the trifolium (shamrock) as its emblem, symbol of harmony, unity, nature, fertility and good luck, as proposed by Michalis Papazoglou. In 1921 and 1922, the Athens-Piraeus FCA organised the first two post-WWI championships, in both of which PPO was declared champion. By that stage, the club had outgrown both the grounds in Patission Street and Amerikis Square, due mainly to its expansion in other sports, and began to look at vacant land in the area of Perivola on Alexandras Avenue as its potential new ground. After long discussions with the Municipality of Athens, an agreement was finally reached and in 1922 Leoforos ("Avenue" in Greek) was granted to the club.