Istisu is a town in the Sharur-Daralayaz uezd of the former Iravan governorate, later in the former Pashali (Azizbeyov, Vayk) district, and currently in the Vayots Dzor province. It is located by the Arpachay, 173 km from the city of Iravan. Although the name of the village was marked in the 13th-century historical sources, the remains of the 7th-century buildings are also available there. In 1961 an extensive construction work was carried out in Istisu which received the status of a city and operates as a resort city. It was marked on the five-verst map of the Caucasus.

The town was solely inhabited by Azerbaijanis: 114 in 1831, 117 in 1886, 163 in 1897, 214 in 1904, 235 in 1914 and 200 Azerbaijanis in 1916. In 1918 the village was invaded by Armenian armed forces and its residents were deported. Following the establishment of Soviet power in present-day Armenia, i.e., in 1920, the surviving Azerbaijanis managed to return to their native lands. The town was solely inhabited by Azerbaijanis: 118 in 1922, 60 in 1926 and 215 Azerbaijanis in 1931. The Armenians removed from Iran and Türkiye in the 1940s were settled there. It was inhabited by 329 Azerbaijanis in 1959, and 627 Azerbaijanis and Armenians in 1970. The Azerbaijani residents of the city were deported from their historical and ethnic lands in November 1988. Currently, the town is inhabited only by Armenians.

The toponym was coined from the name of mineral water naturally coming from the earth in a hot form, and consists of the components “isti” (hot) and “su” (water). It is a hydrotoponym.

The name of the settlement was literally translated into Armenian and changed to “Jermuk”. According to the law “On the administrative-territorial division of the Republic of Armenia” dated 7 November 1995, it was integrated into the administrative territory of the Vayots Dzor province.

Geographical coordinates: latitude: 39°51’ N., longitude: 45°41’ E.