An exarch (; from Ancient Greek ἔξαρχος exarchos, meaning “leader”) was the holder of any of various historical offices, some of them being political or military and others being ecclesiastical.

In the late Roman Empire and early Byzantine Empire, an exarch was a governor of a particular territory. From the end of the 3rd century or early 4th, every Roman diocese was governed by a vicarius, who was titled "exarch" in eastern parts of the Empire, where the Greek language and the use of Greek terminology dominated, even though Latin was the language of the imperial administration from the provincial level up until the 440s (Greek translations were sent out with the official Latin text). In Greek texts, the Latin title is spelled βικάριος (bikarios). The office of exarch as a governor with extended political and military authority was later created in the Byzantine Empire, with jurisdiction over a particular territory, usually a frontier region at some distance from the capital Constantinople.

In the Eastern Christian Churches (Eastern Orthodox, Oriental Orthodox and Eastern Catholic), the term exarch has three distinct uses: a metropolitan who holds the office of exarch is the deputy of a patriarch and holds authority over bishops of the designated ecclesiastical region (thus, a position between that of patriarch and regular metropolitan); or an auxiliary or titular bishop appointed to be exarch over a group of the faithful not yet large enough or organized enough to be constituted an eparchy or diocese (thus the equivalent of a vicar apostolic); or a priest or deacon who is appointed by a bishop as his executive representative in various fields of diocesan administration (in the Byzantine Empire, executive exarchs were usually collecting diocesan revenues for local bishops).