This article discusses Constans, the Roman emperor. For Constans the mythical king of Britain, see: Constans son of Constantine

Bronze coin bearing the profile of Constans

Flavius Julius Constans (AD 320 - January 18, 350), was a Roman emperor who ruled from 337 to 350. Constans was the youngest son of Constantine I the Great and Fausta, Constantine's second wife.

From 337, he was a joint ruler with his brothers Constantius II and Constantine II. Constantine II attempted to take advantage of his youth and inexperience by invading Italy in 340, but Constans defeated Constantine II at Aquileia, where the older brother died.

The writer Julius Firmicus Maternus mentioned that Constans visited Britain in the early months of 343, but did not explain why. The speed of his trip, paired with the fact he crossed the English Channel during the dangerous winter months, suggests it was in response to a military emergency of some kind.

In 350, the general Magnentius declared himself emperor with the support of the troops on the Rhine frontier, and later the entire Western portion of the Roman Empire. Constans lacked any support beyond his immediate household, and was forced to flee for his life. Magnentius' supporters cornered him in a fortification in southeastern Gaul, where he was killed.