Julius Nepotianus (died 30 June 350), sometimes known in English as Nepotian, was a member of the Constantinian dynasty who reigned as a short-lived usurper of the Roman Empire. He ruled the city of Rome for twenty-eight days, before being killed by his rival usurper Magnentius' general Marcellinus.

Roman usurpers were individuals or groups of individuals who obtained or tried to obtain power by force and without legitimate legal authority. Usurpation was endemic during the Roman imperial era, especially from the crisis of the third century onwards, when political instability became the rule.

The first dynasty of the Roman Empire, the Julio-Claudians (27 BC – 68 AD), justified the imperial throne by familial ties, namely with the connection (although only through adoption) with Augustus, the first emperor. Eventually, conflicts within the Julio-Claudian family triggered a series of murders, which led to the demise of the line. Nero died with public enemy status, and following his suicide, a short civil war began, known as the Year of the Four Emperors. The Flavian dynasty started with Vespasian, only to end with the assassination of his second son, Domitian. The 2nd century was a period of relative peace that was marked by the rule of the so-called Five Good Emperors, but the next century would be characterised by endemic political instability, one of the factors that eventually contributed to the fall of the Western Roman Empire.