Urban expansion continued during the Byzantine 4th Century CE a large bath house was built in between the settlement hill and the western city, along the southern side of the Decumanus Maximus

Construction of the large 5 aisled basilica church is built over the earlier (1st or 2nd Century) Roman mausoleum at the western end of the city elaborately decorated with polychrome and glass mosaics in geometric, plant and animal patterns. The earlier Roman mausoleum over which the church was built was reused throughout the Roman and early Byzantine periods and was extended eastwards in the early 4th century becoming associated with early Christian martyrs. 14 high status burials from the 4th to mid 5th century indicate the importance of the church for the city’s Christian elite and the mosaic grave covering for one of these graves names the persons buried below as Valentinianos, Eustathia and Protogenia, who were likely a couple and their daughter.

Construction of the new bath house at the northern city border furnished with beautiful mosaic floors featuring geometric patterns, flowers and vines and a panel that names Herakleides as the donor who paid for the construction of the baths.

Declining Byzantine power and loss of territory in Southern Levant and Egypt during the Byzantine Sassanian War (602 – 628CE) resulted in a gradual decline in prosperity for Gadara. Abandonment of the large Byzantine bath complex during this period may be associated with shortages of fuel and reduction in water supply associated with this loss of prosperity.