Valentinian II (Latin: Flavius Valentinianus; 371 – 15 May 392) was a Roman emperor in the western part of the Roman empire between AD 375 and 392. He was at first junior co-ruler of his brother, was then sidelined by a usurper, and only after 388 sole ruler, albeit with limited de facto powers.

A son of emperor Valentinian I and empress Justina, he was raised to the imperial office at the age of 4 by military commanders upon his father's death. Until 383, Valentinian II remained a junior partner to his older half-brother Gratian in ruling the Western empire, while the East was governed by his uncle Valens until 378 and Theodosius I from 379. When Gratian was killed by the usurper emperor Magnus Maximus in 383, the court of Valentinian in Milan became the center of Italy where several religious debates took place. In 383, Maximus invaded Italy, spurring Valentinian and his family to escape to Thessalonica where they successfully sought Theodosius' aid. Theodosius defeated Maximus in battle and re-installed Valentinian in the West. However, Valentinian soon found himself struggling to break free from the control of general Arbogast. In 392, Valentinian was discovered hanged in his room under unknown circumstances.