The Ecumenical Patriarch of Constantinople (Greek: Οἰκουμενικός Πατριάρχης, romanized: Oikoumenikós Patriárchēs) is the archbishop of Constantinople and primus inter pares (first among equals) among the heads of the several autocephalous churches that comprise the Eastern Orthodox Church. The ecumenical patriarch is regarded as the representative and spiritual leader of the Eastern Orthodox Christians worldwide. The term ecumenical in the title is a historical reference to the Ecumene, a Greek designation for the civilised world, i.e. the Roman Empire, and it stems from Canon 28 of the Council of Chalcedon.

The patriarch's see, the Ecumenical Patriarchate of Constantinople, is one of the most enduring institutions in the world and has had a prominent part in world history. The ecumenical patriarchs in ancient times helped in the spread of Christianity and the resolution of various doctrinal disputes. In the Middle Ages, they played a major role in the affairs of the Eastern Orthodox Church, as well as in the politics of the Orthodox world, and in spreading Christianity among the Slavs. Currently, in addition to the expansion of the Christian faith and the Eastern Orthodox doctrine, the patriarchs are involved in ecumenism and interfaith dialogue, charitable work, and the defense of Orthodox Christian traditions.

Within the five apostolic sees of the Pentarchy, the ecumenical patriarch is regarded as the successor of Andrew the Apostle. The current holder of the office is Bartholomew I, the 270th bishop of that see.

The ecumenical patriarch of Constantinople is first among equals, or first in honor among all Eastern Orthodox bishops, who presides in person—or through a delegate—over any council of Orthodox primates or bishops in which he takes part and serves as primary spokesman for the Orthodox communion especially in ecumenical contacts with other Christian denominations. He has no direct jurisdiction over the other patriarchs or the other autocephalous Orthodox churches, but he, alone among his fellow primates, enjoys the right of convening extraordinary synods consisting of them or their delegates to deal with ad hoc situations and has also convened well-attended pan-Orthodox synods in the last 40 years. His unique role often sees the ecumenical patriarch referred to as the spiritual leader of the Orthodox Church in some sources, though this is not an official title of the patriarch nor is it usually used in scholarly sources on the patriarchate. The Orthodox Church is entirely decentralized: it has no central authority, earthly head, or single bishop in a leadership role. Because it has a synodical system canonically, it is significantly distinguished from the hierarchically organized Catholic Church, whose doctrine is papal supremacy and whose head is the pope. His titles primus inter pares. 'first among equals', and "ecumenical patriarch" are of honor rather than authority, and in fact the ecumenical patriarch has no real authority over churches other than the Constantinopolitan.

The ecumenical patriarch of Constantinople is the direct administrative superior of dioceses and archdioceses serving millions of Greek, Ukrainian, Rusyn and Albanian believers in North and South America, Western Europe, Australia and New Zealand, South Korea, as well as parts of modern Greece which, for historical reasons, do not fall under the jurisdiction of the Church of Greece.^{[citation needed]}

His actual position is patriarch of the Orthodox Church of Constantinople, one of the fourteen autocephalous and several autonomous churches and the most senior (though not oldest) of the four orthodox ancient primatial sees among the five patriarchal Christian centers comprising the ancient Pentarchy of the undivided Church. In his role as head of the Orthodox Church of Constantinople, he is known as the "archbishop of Constantinople, New Rome".

The Ecumenical Patriarchate is also sometimes called the Greek Patriarchate of Constantinople to distinguish it from the Armenian Patriarchate and the extinct Latin Patriarchate, which was created after the Latin capture of Constantinople in 1204, during the Fourth Crusade.

The see of Byzantium, whose foundation was later ascribed to Andrew the Apostle, was originally a common bishopric. It gained importance when Emperor Constantine elevated Byzantium to a second capital alongside Rome and named it Constantinople. The see's ecclesiastical status as the second of five patriarchates was developed by the Ecumenical Councils of Constantinople in 381 and Chalcedon in 451.