Lucius Aelius Sejanus (c. 20 BC – 18 October AD 31), commonly known as Sejanus (), was a Roman soldier and confidant of the Roman Emperor Tiberius. Of the Equites class by birth, Sejanus rose to power as prefect of the Praetorian Guard, the imperial bodyguard, of which he was commander from AD 14 until his execution for treason in AD 31.

While the Praetorian Guard was formally established under Emperor Augustus, Sejanus introduced a number of reforms which saw the unit evolve beyond a mere bodyguard into a powerful and influential branch of the government involved in public security, civil administration and ultimately political intercession; these changes had a lasting impact on the course of the Principate.

During the 20s, Sejanus gradually accumulated power by consolidating his influence over Tiberius and eliminating potential political opponents, including the emperor's son Drusus Julius Caesar. When Tiberius withdrew to Capri in AD 26, Sejanus was left in control of the administration of the empire. For a time the most influential and feared citizen of Rome, Sejanus suddenly fell from power in AD 31, the year his career culminated with the consulship. Amidst suspicions of conspiracy against Tiberius, Sejanus was arrested and executed, along with his followers.