The Eastern Orthodox Church, also called the Orthodox Church, is the second-largest Christian church, with approximately 220 million baptized members. It operates as a communion of autocephalous churches, each governed by its bishops via local synods. The church has no central doctrinal or governmental authority analogous to the head of the Catholic Church—the pope—but the Ecumenical Patriarch of Constantinople is recognized by them as primus inter pares ("first among equals"). As one of the oldest surviving religious institutions in the world, the Eastern Orthodox Church has played a prominent role in the history and culture of Eastern and Southeastern Europe. The Eastern Orthodox Church officially calls itself the Orthodox Catholic Church.

Eastern Orthodox theology is based on the Scriptures and holy tradition, which incorporates the dogmatic decrees of the seven ecumenical councils, and the teaching of the Church Fathers. The church teaches that it is the one, holy, catholic and apostolic church established by Jesus Christ in his Great Commission, and that its bishops are the successors of Christ's apostles. It maintains that it practices the original Christian faith, as passed down by holy tradition. Its patriarchates, reminiscent of the pentarchy, and other autocephalous and autonomous churches, reflect a variety of hierarchical organisation. It recognizes seven major sacraments, of which the Eucharist is the principal one, celebrated liturgically in synaxis. The church teaches that through consecration invoked by a priest, the sacrificial bread and wine become the body and blood of Christ. The Virgin Mary is venerated in the Eastern Orthodox Church as the God-bearer, honored in devotions.

The churches of Constantinople, Alexandria, Jerusalem, and Antioch—except for some breaks of communion such as the Photian schism or the Acacian schism—shared communion with the Church of Rome until the East–West Schism in 1054. The 1054 schism was the culmination of mounting theological, political, and cultural disputes, particularly over the authority of the pope, between those churches. Before the Council of Ephesus in AD 431, the Church of the East also shared in this communion, as did the various Oriental Orthodox Churches before the Council of Chalcedon in AD 451, all separating primarily over differences in Christology.

The Eastern Orthodox Church is the primary religious denomination in Russia, Ukraine, Romania, Greece, Belarus, Serbia, Bulgaria, Moldova, Georgia, North Macedonia, Cyprus, and Montenegro, and there are significant minorities in Kazakhstan, Germany, Spain, Bosnia and Herzegovina, the United States, and Uzbekistan. The majority of Eastern Orthodox Christians live in Southeastern and Eastern Europe, and Siberia. Roughly half of Eastern Orthodox Christians live in the post-Soviet states, mostly Russia. There are also communities in the former Byzantine regions of Africa, the Eastern Mediterranean, and in the Middle East, which are decreasing due to forced migration driven by increased religious persecution. Eastern Orthodox communities outside Eastern Europe, including those in North America, Western Europe, and Australia, have been formed through diaspora, conversions, and missionary activity.