Patr. (649–59), author, and liturgical reformer. He was born in Kuplana (on the Greater Zab, in Adiabene), the son of a prominent landowner named Bastomag. He became a monk of the nearby Monastery of Beth ʿAbe, but was appointed bp. of Nineveh (ca. 629), and a decade or so later, metropolitan of Arbela, finally to become cath. in 649. He is credited with many liturgical reforms, though the nature and extent of these is unclear; it seems that, among other things, it was he who limited the number of Anaphoras in use to the present three. In his ‘Catalogue’ ʿAbdishoʿ attributes to him the arrangement of the Ḥudrā, the Baptismal Liturgy, and rite for the Consecration of a new church; this attribution is also found in the mss. of all these, but the emergence of these liturgical books is likely to have been much more complex (in the case of the Baptismal Service, the mss. record a further reform by the Cath. Eliya  III [1176–90]). A set of Questions and Answers on liturgical topics (in ms. Mingana Syr. 566) is also attributed to him, but Van Unnik, who edited them, showed that they must belong to a later date, and he allocated them to Ishoʿyahb IV.

Ishoʿyahb III is much more certainly the author of a long ‘Life of Ishoʿsabran’, a martyr of the late Sasanian Empire (ed. Chabot, with French summary), and of a collection of 106 Letters (ed. R. Duval, with LT). The Letters are arranged chronologically in three groups, those written while he was bp. (52), while he was metropolitan (32), and while he was cath. (22), though the chronological allocation in some cases cannot be correct. These convey a great deal of information about the life of the Ch. of E. during the period of the transition from Persian to Arab rule. (The edition by P. Scott-Moncrieff [1904] of Ishoʿyahb’s letters written while he was bp. omits Letters 1 and 12 in Duval’s edition; he gives handy brief summaries in English of each letter. His projected full translation never appeared).