Gate 13 (Greek: Θύρα 13), is the name of the ultras group of the Greek multi-sports club Panathinaikos A.O. Officially founded on November 19, 1966, Gate 13 is the oldest supporters' union in Greece. Gate 13 is one of the most powerful ultras group in Europe and has members from all over Greece and Europe, while over the years it has become a part of the club by affecting club decisions and by following the club on all possible occasions.

Before the foundation of Gate 13 there were various football supporters' clubs all around Greece. The first of these clubs was founded in October 1952 and was called S.F.O.P. (Greek: Σ.Φ.Ο.Π., Σύλλογος Φιλάθλων Οπαδών Παναθηναϊκού, Syllogos Filathlon Opadon Panathinaikou, Panathinaikos Supporters Club) and stayed active for an unknown amount of time.

Towards the end of the 50s, supporters started getting organized as far as the home and away match support. They started attending the matches regularly and always met at the 13th gate of the Leoforos Alexandras Stadium. Gate 13 was also a place for the poorest fans and supporters of the team, with the seats not being numbered, unlike the rest of the stadium and the vip seats.

In order to organize trips for away matches and other events, the various fan clubs used to write in the two newspapers most read by Panathinaikos fans of the time, Athlitiki Iho (Greek: Αθλητική Ηχώ, Athletic Echo) and Panathinaika Nea (Greek: Παναθηναϊκά Νέα, Panathenaic News).

By 1966, there were a number of supporters' clubs, all named by the area where they were based. Notable clubs included: Ampelokipi, Zografou, Patissia, Gizi, Petralona, Cholargos, Peristeri, Patras and many more, most of which are still active today. Despite the lack of a universal name, the clubs would often meet to discuss various things including the team's performance and management, various trips for away matches and securing tickets for the home matches. Since 1962–63 there had been thoughts of founding a universal club under the name of the stadium's gate where the team's most devoted supporters met, but at the time there were many difficulties. Slowly the name Gate 13 was starting to get recognized and respected from players and fans.

Then, in 1966, after much effort, the "Syndesmos Filon Panathinaikou Athlitikou Omilou "I Thyra 13"" (Σύνδεσμος Φίλων Παναθηναϊκού Αθλητικού Ομίλου "Η Θύρα 13", Fan Club of Panathinaikos Athletic Club "Gate 13") first operated on November 19, 1966, on 68, Kolonos Str in Athens. At the time, it was a club like all the rest, but the difference was that its name would become a major part of Panathinaikos' history. On November 27, 1966, a few days after the foundation of Gate 13, a bus carrying Panathinaikos fans from Athens to Veroia crashed, resulting in the death of two Panathinaikos fans, Giorgos Koskoros (Greek: Γιώργος Κόσκορος) and Dimitris Sarantakos (Greek: Δημήτρης Σαραντάκος) who was one of the founding members of Gate 13.

In 1967 the supporters' clubs of Panathinaikos were increasing in number in Athens and rest of Greece. In 1967, the Greek military junta of 1967–1974 was established and closed many of them.

So did Gate 13, only to reopen in 1968 with new offices on 39, Sokratous Srt in the center of Athens. Many other clubs followed shortly. At that time, the situation inside the stadiums in Greece was very calm with fights only occasionally breaking out.