Lucius Aelius Seianus (20 BCE – October 18, CE 31), commonly known as Sejanus, is an ambitious soldier, friend and confidant of the Roman Emperor Tiberius.

An equestrian by birth, Sejanus rises to power as prefect of the Roman imperial bodyguard, known as the Praetorian Guard, of which he is commander from CE 14 until his death in CE 31.

While the Praetorian Guard had been formally established under Emperor Augustus, Sejanus introduces a number of reforms which see 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; changes which will have a lasting impact on the course of the Principate.

During the 20s, Sejanus gradually accumulates power by consolidating his influence over Tiberius and eliminating potential political opponents, including the emperor's son, Drusus Julius Caesar.

When Tiberius withdraws to Capri in 26, Sejanus is left in control of the entire state mechanism as de facto ruler of the empire.

For a time the most influential and feared citizen of Rome, Sejanus suddenly falls from power in 31, the year his career culminates with the consulship.

Amidst suspicions of conspiracy against Tiberius, Sejanus is arrested and executed, along with his followers.