Adiabene (Syriac: Hadyab ܚܕܝܐܒ) was a metropolitan province of the Assyrian Church of the East between the 5th and 14th centuries, with more than fifteen known suffragan dioceses at different periods in its history. Although the name Hadyab normally connoted the region around Erbil and Mosul, the boundaries of the East Syrian metropolitan province went well beyond the Erbil and Mosul districts. Its known suffragan dioceses included Beth Bgash (the Hakkari region of eastern Turkey) and Adarbaigan (the Ganzak district, to the southeast of Lake Urmi), well to the east of Adiabene proper.
The bishop of Erbil became metropolitan of Adiabene in 410, responsible also for the six suffragan dioceses of Beth Nuhadra (ܒܝܬ ܢܘܗܕܪܐ), Beth Bgash, Beth Dasen, Ramonin, Beth Mahqart and Dabarin. Bishops of the dioceses of Beth Nuhadra, Beth Bgash and Beth Dasen, which covered the modern ʿAmadiya and Hakkari regions, were present at most of the early synods, and these three dioceses continued without interruption into the 13th century. The other three dioceses are not mentioned again, and have been tentatively identified with three dioceses better known under other names: Ramonin with Shenna d'Beth Ramman in Beth Aramaye, on the Tigris near its junction with the Great Zab; Beth Mahrqart with Beth Qardu in the Nisibis region, across the Tigris from the district of Beth Zabdaï; and Dabarin with Tirhan, a district of Beth Aramaye which lay between the Tigris and the Jabal Hamrin, to the southwest of Beth Garmaï.
By the middle of the 6th century there were also dioceses in the province of Adiabene for Maʿaltha (ܡܥܠܬܐ) or Maʿalthaya (ܡܥܠܬܝܐ), a town in the Hnitha (ܚܢܝܬܐ) or Zibar district to the east of ʿAqra, and for Nineveh. The diocese of Maʿaltha is first mentioned in 497, and the diocese of Nineveh in 554, and bishops of both dioceses attended most of the later synods.
Erbil, the chief town of Adiabene, lost much of its former importance with the growth of the city of Mosul, and during the reign of the patriarch Timothy I (780–823) the seat of the metropolitans of Adiabene was moved to Mosul. The dioceses of Adiabene were governed by a 'metropolitan of Mosul and Erbil' for the next four and a half centuries. Around 1200, Mosul and Erbil became separate metropolitan provinces. The last known metropolitan of Mosul and Erbil was Tittos, who was appointed by Eliya III (1175–89). Thereafter separate metropolitan bishops for Mosul and for Erbil are recorded in a fairly complete series from 1210 to 1318.