A.E.K. (Greek: AEK [ˈaek], formally Αθλητική Ένωσις Κωνσταντινουπόλεως; Athlitikí Énosis Konstantinoupόleos, Athletic Union of Constantinople), known as A.E.K., is the most successful Greek multi-sport club, based in Nea Filadelfeia, Athens, Attica. The club is more commonly known in European competitions as A.E.K. Athens.

Established in Athens, in 1924, by Greek refugees from Constantinople, after the 1919–22 Greco-Turkish war and the subsequent population exchange between Greece and Turkey, it maintains more than 30 sports departments (football, basketball, handball, volleyball, futsal, etc.), under the control of its amateur sports arm, Amateur AEK (Greek: Ερασιτεχνική ΑΕΚ; Erasitechnikί AEK), with noteworthy departments, such as its handball team, which is the best Greek handball club, in terms of European achievements, having obtained 1 EHF European Cup in 2021 and having also reached the final in 2018 and in 2025 and the semi-finals in 2019. AEK sports club is best known for its professional football team, which has made some notable wins in European competitions and reached the quarter-finals of the UEFA Champions League in 1969 and the semi-finals of the UEFA Europa League in 1977, having also won 13 Greek Championships, 16 Greek Cups, 1 Greek League Cup, 2 Greek Super Cups and also for its professional basketball team which has won 8 Greek Basketball Leagues, 5 Greek Basketball Cups, 2 FIBA Saporta Cups, 1 Basketball Champions League and 1 FIBA Intercontinental Cup.

The large Greek population of Constantinople, not unlike those of the other Ottoman urban centres, continued its athletic traditions in the form of numerous athletic clubs. Clubs such as Énosis Tataoúlon (Ένωσις Ταταούλων) and Iraklís (today Kurtuluş S.K.) (Ηρακλής) from the Tatavla district, Mégas Aléxandros (Μέγας Αλέξανδρος) and Ermís (Ερμής) of Galata, and Olympiás (Ολυμπιάς) of Therapia existed to promote the Hellenic athletic and cultural ideals. These were amongst a dozen Greek-backed clubs that dominated the sporting landscape of the city in the years preceding World War I. After the war, with the influx of mainly French and British soldiers to Constantinople, many of the city clubs participated in regular competition with teams formed by the foreign troops. Taxim, Pera, and Tatavla became the scene of weekly competitions in football, athletics, cycling, boxing and tennis.

Ermís, one of the most popular clubs, was formed in 1875 by the Greek community of the city.

Another club of Pera (Galata), known as Pera, since the mid-1880s, forced to change its name to Pera Sports Club, and then Beyoğlu S.K., in 1923. Many of its athletes, with those of the other sporting clubs, fled during the population exchanges at the end of the Greco-Turkish War, and settled mainly in Athens and Thessaloniki.

In 1924, the founders of AEK - a group of Constantinopolitan refugees (among them many athletes from the Pera Sports Club and the other Constantinopolitan clubs) - met at the athletic shop "Lux" of Emilios Ionas and Konstantinos Dimopoulos on Veranzerou Street, in the center of Athens, and created AEK. Their intention was to create a club that provided athletic and cultural diversions for the thousands of predominantly Constantinopolitan and Anatolian refugees who had settled in the new suburbs of Athens (including Nea Filadelfeia, Nea Ionia, Nea Chalkidona, Nea Smyrni).

AEK's first president, Konstantinos Spanoudis (1871–1941), a journalist and associate of the Prime Minister Eleftherios Venizelos, petitioned the government to set aside land for the establishment of a sports ground. In 1926, land in Nea Filadelfeia that was originally set aside for refugee housing, was donated as a training ground for the refugees' sports activities. AEK began using the ground for training, albeit unofficially.

In 1930, the property where AEK trained was officially signed over to the club. Venizelos soon approved the plans to build what was to become AEK's home ground for the next 70 years, the Nikos Goumas Stadium. The liberal politician was present at the inauguration of the stadium.