A Roman or civil diocese (Latin: dĭœcēsĭs, from the Greek: διοίκησις, "administration") was one of the administrative divisions of the later Roman Empire, starting with the Tetrarchy. It formed the intermediate level of government, grouping several provinces and being in turn subordinated to a praetorian prefecture.

The earliest use of 'diocese' as an administrative unit was in the Greek-speaking East. Three districts— Cibyra, Apamea and Synnada— were added to the province of Cilicia in the time of Cicero, who mentions the fact in his familiar letters (EB 1911). The word 'diocese', which at that time was equivalent to a tax-collecting district, came to be applied to the territory itself.

The reorganization of the Roman Empire known as Tetrarchy was initiated by Emperor Diocletian in the 290s. He divided the existing provinces into smaller, more compact and easily controllable units, with a greatly increased bureaucracy. The provinces were in turn grouped into twelve dioceses, each headed by a vicarius [dioeceseos], i.e. a vicar (representative) of the praetorian prefect, the Empire's chief civil and military official and senior aide to an emperor. Under the tetrarchic system, each of the two senior emperors (Augusti) had a praetorian prefect. The largest diocese, Oriens, included sixteen provinces, and the smallest, Britannia, comprised only four provinces.