Osroes I of Parthia ruled the Parthian Empire c. 109–129. He succeeded his brother Pacorus II. For the whole of his reign he contended with the rival king Vologases III based in the east of Parthia.

Orsoes invaded Armenia and placed first his nephew Exedares, and then his brother Parthamasiris on the Armenia throne. This enchroachment on the traditional Roman sphere of influence — the two great empires had shared hegemony over Armenia since the time of Nero some 50 years earlier — led to a war with the emperor Trajan.

In 113 Trajan invaded Parthia, marching first on Armenia. In 114 Parthamasiris surrendered and was killed. Trajan annexed Armenia to the Roman empire. Then he turned south into Parthia itself, taking the cities of Babylon, Seleucia and finally the capital of Ctesiphon in 116. He deposed Osroes I and put his own puppet ruler Parthamaspates on the throne. In Mesopotamia Osroes' brother Mithridates IV and his son Sanatruces II took the diadem and fought against the Romans, but Trajan marched southward to the Persian Gulf, defeated them, and declared Mesopotamia a new province of the empire. Later in 116, he crossed the Khuzestan mountains into Persia and captured the great city of Susa.

Following the death of Trajan and Roman withdrawal from the area, Osroes easily defeated Parthamaspates and reclaimed the Persian throne. Hadrian acknowedged this fait accompli, recognized Osroes, Parthamaspates king of Osroene, and returned Osroes' daughter who had been taken prisoner by Trajan.

It seems that the conflict with Rome had weakened Osroes and strengthened his vival Vologases — from about 121 there are few coins of the former and many of the latter.

References

• Dio Cassius, lxviii, 17–33.
• John Malalas, Chronographia.
• Aelius Spartianus, Vita Hadrian, v, 13.
• Pausanias, v, 12.