The Roman–Parthian War of 161–166 was fought between the Roman and Parthian Empires over Armenia and Upper Mesopotamia. It concluded in 166 after the Romans made successful campaigns into Lower Mesopotamia and Media and sacked Ctesiphon, the Parthian capital.

Vologases IV was King of Kings of the Parthian Empire from 147 to 191. He was the son of Mithridates V. Vologases spent the early years of his reign re-asserting Parthian control over the Kingdom of Characene. From 161 to 166, he waged war against the Roman Empire; although initially successful, conquering Armenia and Syria, he was eventually pushed back, briefly losing control of the Parthian capitals of Seleucia and Ctesiphon to the Romans. The Romans suffered heavy losses from a plague erupting from Seleucia in 166, forcing them to withdraw. The war ended soon afterward, with Vologases losing most of northern Mesopotamia to the Romans. He died in 191 and was succeeded by his son Vologases V.