In the Byzantine Empire, an exarch ( /ˈɛksɑrk/; Ancient Greek: ἔξαρχος exarchos) was governor with extended authority of a province at some remove from the capital Constantinople. The prevailing situation frequently involved him in military operations.

In the Eastern Christian Churches (Orthodox, Oriental Orthodox and Eastern Catholic), the term exarch has two distinct uses: the deputy of a patriarch, or a bishop who holds authority over other bishops without being a patriarch (thus, a position between that of patriarch and metropolitan); or, a bishop appointed over a group of the faithful not yet large enough or organized enough to be constituted an eparchy/diocese (thus the equivalent of a vicar apostolic).

In the civil administration of the Byzantine (Eastern Roman) Empire the exarch was, as stated above, the viceroy of a large and important province. The Exarchates were a response to weakening imperial authority in the provinces and were part of the overall process of unification of civil and military offices, initiated in early form by Justinian I, which would lead eventually to the creation of the Theme system by Emperor Heraclius.