chief administrator of the Roman Empire for the emperor Tiberius, alleged murderer of Tiberius' only son, Drusus Caesar, and suspect in a plot to overthrow Tiberius and become emperor himself.

Sejanus was related through his mother to the distinguished senatorial family Cornelii Lentuli. In AD 14, upon the accession of Tiberius, he joined his father, Seius Strabo, as prefect of the Emperor's household troops, the Praetorian Guard, and became sole prefect a year or two later. As prefect he gained the complete confidence of the Emperor.

After the death of Drusus (AD 23), he began systematically to attack the position of Drusus' mother, Agrippina the Elder (Agrippina, Vipsania), whose sons were Tiberius' probable heirs. In 25, Sejanus was refused Tiberius' permission to marry Drusus' widow, Livilla, who may have been Sejanus' accomplice in the poisoning of her husband. In 27, however, Sejanus persuaded the Emperor to take up residence on Capreae (modern Capri) and in 29 succeeded in having Agrippina and her son Nero exiled.

As consul (31), with the prospect of attaining the great administrative prerogatives of tribunician power, Sejanus fell under the suspicion of the emperor. Tiberius, repeatedly warned by his sister-in-law Antonia of Sejanus' designs, had him arrested and executed. The Senate and the populace rejoiced as his body was dragged through the streets, and a long reign of terror ensued against his adherents.