It is difficult to trace the roots of the city to any one founder. Some believe that Alexander the Great was the founder of Jerash, which emerged from the lands he had conquered in 334 BCE.  Other legends link the origin to Ptolemy II of Egypt in the middle of the third century BCE. The first historical reference to the town is at the end of the second century BCE when the historian Josephus recorded it as the location where Theodosus, the "Tyrant" of Philadelphia (modern day Amman) had hidden his treasure. It was soon after this event that Theodosus lost Jerash to Alexander Jannaeus, a Jewish high priest and ruler.

Of the original Hellenistic town there are no visible structures remaining today, however, many archaeological excavations uncovered artifacts from this early period of Jerash’s habitation. Aside from the Temple of Zeus, mentioned in historical writings, there is very little knowledge of the types of buildings that existed during the pre-Christian era. The town stayed in Jewish hands until 63 BCE when the Roman took the area. At that time Pompey overran the region and divided it into provinces. The Romans knew Jerash as Gerasa which was the Hellenized version of the Semitic name, Garshu, previously used for a village that predated Alexander’s conquest of the region.