City on the Khabur, located in the eastern part of ancient Osrhoene and today straddling the Turkish-Syrian border, the Syrian part now bearing the name Raʾs al-ʿAyn and the Turkish part Ceylanpınar (earlier Resülayn). Its Syriac and Arabic names, meaning ‘spring-head’ in both cases, refer to the large number of springs in the area, from which the Khabur draws much of its water. The biblical city of Resen (Gen. 10:12; Pesh.: Rasan) was identified with Reshʿayna by Ephrem (Commentary on Genesis, ed. R. M. Tonneau [CSCO 152], 65.25–6) and in the later Syr. exegetical tradition (as well as in the Palestinian Targumim). In classical times it was also known as Theodosiopolis after Emperor Theodosius I (379–95), who according to the Chronicle of Edessa ‘founded’ the city in 380/1 (ed. Guidi, 5.17–8) and according to Malalas granted it municipal status in 383. The city, which was taken by the Arabs in 640, saw brief occupations by the Byzantines in 943 and by the Franks under Count Joscelin I of Edessa in around 1129. It fell into insignificance in the modern period, so that the present-day twin city of Raʾs al-ʿAyn-Ceylanpınar is largely a modern foundation created in the late 19th cent.

Among the early bishops of Reshʿayna mention may be made of Andrew, banished for his opposition to the ‘Henoticon’ in 484, Peṭros, a correspondent of Severus of Antioch who was exiled by Justin I in 519, and ʾSWL (Asylios/Ascholios?), whom Sergios of Reshʿayna denounced before the Chalcedonian Patr. Ephrem of Antioch in ca. 535. After the Islamic conquest, we hear of the Syr. Orth. bp. Gabriel in 667. The synod at which Athanasios II of Balad was elected patr. was held in Reshʿayna in 684. The names of nine bishops of Reshʿayna between 724 and 986 are known from the Chronicle of Michael I Rabo. In the 11th and early 12th cent. the see of Reshʿayna is usually found united with that of Mardin.

Among the E.-Syr. authors mentioned by ʿAbdishoʿ bar Brikha are Daniel of Reshʿayna (ca. 550) (Assemani, BibOr III/1, 223) and Shalliṭa of Reshʿayna, the latter of whom is said in mss. of his work to have been a bp. Reshʿayna is mentioned as a suffragan of Nisibis in a list of E.-Syr. dioceses in 1007/8.

An important monastery in the neighbourhood of Reshʿayna is known under different names: 1. Speqlis/ʾEspeqlis, i.e., ‘Specula’ or ‘Watchtower’ (mss. Brit. Libr. Add. 14,430 and 12,135[I], two Reshʿayna, ‘who never made a blotted taw’; see Wright, Catalogue, vol. I, 15b–16b and 24b–26a; comp. ibid., 9a–10a); 2. Asphulos/Asphuloye/Saphylos (Michael Rabo); and 3. Pgimto (Bar ʿEbroyo, ‘Chron. Eccl.’, ed. Abbeloos and Lamy, I.281: Dayro da-Pgimto ʾawkit d-SPWLWS, which is mentioned here as the monastery of origin of Patr. Severos bar Mashqo). Barsoum claims that it was built in the 5th cent. and reduced to ruins shortly before 1203, and that it produced two patriarchs and eleven bishops.

Two monks who are regularly quoted in the margin of Masora mss., Ṭubhono and Sobho, may have belonged to the nearby Monastery of Qarqaphto. Their fame also spread to the Ch. of E., as Bar Bahlul mentions them in his Lexicon (ed. R. Duval, 1363–4), specifically linking them to Reshʿayna and crediting them with an important role in the ‘tradition (mašlmonutho) of the (two) Testaments’. Whether this monk Sobho (whom Bar Bahlul also calls Sanṭo) might be the same as the early-8th-cent. Sobho of the ‘Watchtower’, mentioned above, remains an open question (Loopstra argues for their identity).

On the stretch of the Khabur between Reshʿayna and Hassaka to the south are thirty-seven villages settled by refugees from the Hakkari region, mostly members of the Ch. of E., who arrived here via Iraq in 1930s.