A Greek Orthodox Church (Greek: Ἑλληνορθόδοξη Ἐκκλησία, romanized: Ellinorthódoxi Ekklisía, IPA: [elinorˈθoðoksi ekliˈsia]) is any of three classes of Christian churches, each associated in some way with Greek Christianity, Levantine Arabic-speaking Christians or more broadly the rite used in the Eastern Roman Empire:

Historically, the term "Greek Orthodox" has been used to describe all Eastern Orthodox churches, since the term "Greek" can refer to the heritage of the Byzantine Empire. During the first eight centuries of Christian history, most major intellectual, cultural, and social developments in the Christian Church took place in the Byzantine Empire or its sphere of influence, where the Greek language was widely spoken and used for most theological writings. The empire's capital, Constantinople, was an early important center of Christianity, and its liturgical practices, traditions, and doctrines were gradually adopted throughout Eastern Orthodoxy, still providing the basic patterns of contemporary Orthodoxy. Thus, Eastern Orthodox came to be called "Greek" Orthodox in the same way that Western Christians came to be called "Roman" Catholic. However, the appellation "Greek" was abandoned by the Slavic and other Eastern Orthodox churches as part of their peoples' national awakenings, beginning as early as the 10th century A.D. Thus, by the early 21st century, generally only those churches most closely tied to Greek or Byzantine culture and ethnicity were called "Greek Orthodox" in common parlance.

Greek Orthodoxy has also been defined as a religious tradition rooted in preserving the Greek identity. In 2022, U.S. government estimated that 81–90% of the population of Greece identified as Greek Orthodox.

The Greek Orthodox churches are descendants of churches which the Apostles founded in the Balkans and the Middle East during the first century A.D., as well as maintainers of many ancient church traditions.