During the 6 day Battle of Yarmouk near Gadara, the army of the Byzantine empire was defeated by the Muslim Rashidan Caliphate. Collapse of Byzantine power in the Levant after the battle meant Gadara passed to Muslim (Rashid and then later Umayyad) control. Islamic influence at Gadara can be seen archaeologically from the construction of a Mosque and bazaar, the latter utilising and adapting the earlier colonnade of the main decumanus maximus street. Archaeological evidence of repair and continued use of the Christian churches well in to the Umayyad period shows that the city remained prosperous and whilst Gadara was part of a rapidly expanding Islamic world, change for the citizens was gradual.

A huge earthquake caused massive destruction across the city and the wider region. Many of the Byzantine monuments collapsed including the ecclesiastical buildings on the Roman terrace. The earthquakes impact on the wider region included shifting and collapse of the qanat aqueduct tunnels severing the water supply connections that supported the city in the arid climate of Northern Jordan
Without a secure water supply the city went into rapid decline. Archaeological evidence suggests that there was some limited re-occupation of the city including the rebuilding of the five aisled basilica on a smaller scale as a mosque. However, limited archaeological excavation of areas of the city away from the decumanus maximus street and the settlement hill make determining the degree of occupation and population size in early Islamic Gadara after 749CE difficult.
Some parts of the city did survive and continue to be used. The disused Byzantine bath complex was subdivided for use as stables and the five aisled basilica was rebuilt although reduced in size although collapsed columns and masonry found in situ show large parts of the city were not reused.

The five aisled basilica church rebuilt after the 749 earthquake was converted from a Christian church to a mosque indicating that there was still a sizable population living in and around the area.

Battle of Hattin. Saladin (Ṣalāḥ ad-Dīn Yūsuf ibn Ayyūb) defeats Christian forces at Hattin, to the west of Galilee to gain control of most of the territory of the Kingdom of Jerusalem, including the area around Gadara.

The village is recorded in an Arabic documentary source as “Mukais” probably deriving from a term for border place or customs house. Whilst we have little record of the settlement from this point on the continued use of the same settlement name indicates a continuity of settlement here to the modern day. The change from Mkeis to the more formal “Umm Qais” is due to the Arabization [ta’reeb] of Latin or non-Arabic names that took place in Jordan during the twentieth century.