Sejanus (Lucius Aelius Sejanus)sĭjāˈnəs, d. A.D. 31, Roman statesman; son of Sejus Strabo, Praetorian prefect. When his father went to Egypt as governor, he succeeded to the command of the Praetorian Guards and obtained great ascendancy over Emperor Tiberius. He was suspected of conspiring (A.D. 23) with Livilla in a successful plot to poison her husband, the emperor's son Drusus. He obtained (A.D. 29) the arrest of Agrippina the Elder. Sejanus was put to death by Tiberius, who feared that he was plotting against him.