Dio Cassius says that his praenomen was Lucius. Tacitus (Tac. Ann. 4.1, &c.) is our chief authority for the history of this infamous instrument of Tiberius. Sejanus was born at Vulsinii, in Etruria : he was the son of Seius Strabo, a Roman eques, who was commander of the praetorian troops at the close of the reign of Augustus and the commencement of that of Tiberius. Velleius Paterculus (2.127) says that lie was of illustrious descent on the maternal side ; and Lipsias conjectures that his mother was of the Junia Gens, because Junius Blaesus, proconsul of Africa, was the maternal uncle of Sejanus (Tac. Ann. 3.72). Rumour accused him of selling himself, when a young man, to the lust of Apicíus, a rich debauchee (D. C. 57.19). Sejanus ultimately gained such influence over Tiberius, that this suspicious man, who was close and reserved to all mankind, opened his bosom to Sejanus, and made him his confidant. Sejanus had a body capable of enduring fatigue, and a mind capable of the boldest designs : he concealed his own thoughts, and was a calumniator of others ; he could fawn and crouch to power, though he was insolent to those below him; to the world he put on the appearance of moderation, but his greediness had no bounds; and to accomplish his purposes he could be magnificent and profuse, as well as laborious and vigilant. Such was the character of the man who for many years governed Tiberius.

In the year in which Augustus died, A. D. 14, Sejanus was made the colleague of his father in the command of the Praetorian bands, and was sent by Tiberius to accompany his son Drusus, in his visit to the mutinous legions in Pannonia (Tac. Ann. 1.24). Upon his father being sent as governor to Egypt, Sejanus had the sole command of the Praetorian cohorts. When Agrippina, the wife of Germanicus, by her heroic resolution, had prevented the bridge over the Rhine from being destroyed, and thus secured to the Roman legions their retreat from the east bank of the river, the suspicious temper of Tiberius, who was afraid of a woman of such a stamp, was still further irritated by the insinuations of Sejanus, who sowed the seeds of hatred in the mind of Tiberius, to the end that they might ripen in due time. The ambitious designs of Sejanus began to be suspected by the Romans when Tiberius betrothed the daughter of Sejanus to Drusus, the son of Claudius, who was afterwards emperor. The marriage was prevented by the untimely death of the youth (Sueton. Claud. 27). In A. D. 22 the theatre of Pompeius was burnt (Tac. Ann. 3.72), on which occasion Sejanus received the thanks of Tiberius, for preventing the conflagration from spreading further. Seneca (Ad Marciam, 22) states, that when a statue of Sejanus was decreed to be placed in the building which Tiberius restored, Cremutius Cordus exclaimed that the theatre was now really ruined.

Sejanus was the person who advised that the Praetorian cohorts, which had hitherto been disposed in various parts of the city, should be stationed in one camp (Tac. Ann. 4.2), a measure which was entirely opposed to the system of Augustus (Sueton. Aug. 49). He urged that the troops would be less manageable if they were scattered; that they would be more efficient for all emergencies if they were in one place; and would be more removed from the dissipation of the city. But they were not removed from the city; they were stationed close to it; and they afterwards controlled Rome, as masters, whenever the occasion came. The object of Sejanus was to make himself popular with the soldiers. He appointed the centurions and tribunes : he gave posts of honour and emolument to his creatures and favourites; and Tiberius, the most suspicious of men, had such confidence in the praefect, that he called him his associate in the labours of administration, and allowed his busts to stand in the theatres and fora, and even to be placed in the principia of the legions. The cunning tyrant was completely infatuated with a man whose object was to destroy his master.

There were many obstacles between Sejanus and the imperial power, but he set about removing them. Drusus, the son of Tiberius, who was of a hasty temper, had given Sejanus a blow, in a dispute with him; for this version of the story is more probable than that which makes Sejanus give the blow (Tac. Ann. 4.3, and the note of Lipsius). Sejanus revenged himself by debauching Livia or Livilla, the sister of Germanicus, and the wife of Drusus; and he encouraged her to the murder of her husband, by promising her marriage and a participation in the imperial power to which he aspired. To show that he was in earnest, Sejanus divorced his wife Apicata. The crime was delayed until there was a fitting opportunity, and Drusus was poisoned by Sejanus (Tac. Ann. 4.8, 10, A. D. 23). Sejanus asked the permission of Tiberius to marry Livia, but the emperor rejected his petition, though in studied language, and in terms that did not take away all hope. Sejanus saw that it was time to act with caution; he persuaded Tiberius to retire from Rome into privacy, hoping that he should thus gradually centre all the administration in himself. Agrippina, the wife of Germanicus, was now a widow; and Sejanus, who feared and hated her, instilled into Tiberius suspicions that she had a party at Rome. Agrippina, being weary of her widowed state, asked Tiberius to allow her to marry again; but the emperor gave no answer to her urgent entreaties. Sejanus seized the occasion to make Agrippina suspicious of the designs of Tiberius, and his agents persuaded her that the emperor designed to take her off by poison. Agrippina, who was not a woman to conceal her thoughts, plainly showed Tiberius, at a banquet, that she suspected his designs; and the emperor uttered words which imported that if he were suspected of wishing to poison her, it could not be surprising if he let her feel his resentment. An accident increased the credit of Sejanus, and confirmed the confidence of Tiberius. The emperor, with Sejanus and others, was feasting in a natural cave, between Amyclae, which was on the sea coast, and the hills of Fundi. The entrance of the cave suddenly fell in, and crushed some of the slaves; and all the guests, in alarm, tried to make their escape. Sejanus, resting his knees on the couch of Tiberius, and placing his shoulders under the falling rock, protected his master, and was discovered in this posture by the soldiers who came to their relief. After Tiberius had shut himself up in the island of Capreae, Sejanus had full scope for his machinations, and the death of Livia, the mother of Tiberius (A. D. 29), was followed by the banishment of Agrippina and her sons Nero and Drusus.

Tiberius at last began to suspect the designs of Sejanus; perhaps he had suspected them for some time, but he had duplicity enough to conceal his suspicions. Josephus states that Antonia, his sister-in-law, informed him by letter of the ambitious views of Sejanus. Tiberius felt that it was time to rid himself of a man who was almost more than a rival. To cover his schemes and remove Sejanus from about him, Tiberius made him joint consul with himself, in A. D. 31; and gave a pontificate to him and his son. Still he would not let Sejanus come to him in his retreat, and while he still amused him with the hopes of Livia's marriage, he was plotting his ruin. In the mean time Tiberius strengthened himself by making Caligula a pontifex Augusti and intimating that he was to be his successor. Sejanus saw the danger coming, but he was unable to prevent it. Tiberius, acting with his usual duplicity, gave Sejanus reason to believe that he was going to associate him with himself in the tribunitian authority; but at the same time he sent Sertorius Macro to Rome, with a commission to take the command of the praetorian cohorts. Macro, after assuring himself of the troops, and depriving Sejanus of his usual guard, produced a letter from Tiberius to the senate. Tiberius expressed himself in his usual perplexed way, when he wished his meaning to be inferred without being declared in explicit terms. The meaning was clear enough; he was afraid of Sejanus, and wished to be secured against him. Sejanus, who was present, had received the usual fawning submission of the servile senate, so long as they thought that the letter of Tiberius was going to announce new honours for him. When it was read, there was not a man among them to give him a word of consolation or show him a sign of respect. The consul Regulus conducted him to prison, and the people, who would have declared him emperor, if the word had been given to them, loaded him with insult and outrage. His statues were pulled down before his face. The senate on the same day decreed his death, and he was immediately executed. His body was dragged about the streets, and finally thrown into the Titer; or rather, says Seneca (De Tranquillitate, 100.11), there scarcely remained a fragment of it for the executioner to drag to the river.

Many of the friends of Sejanus perished at the same time, among whom was probably his uncle Junius Blaesus. His surviving son and a daughter shared his fate. The daughter was probably the child who had been betrothed to Drusus, the son of Claudius. The girl was so ignorant of what was going on that she frequently asked why they were dragging her along, that she would never do so any more, and would consent to a whipping. The writers of the time stated that it was a thing unheard of for a virgin to be capitally punished by the triumviri, and accordingly she was ravished by the executioner before she was put to death. (Tac. Ann. 5.9.)

Apicata, the divorced wife of Sejanus, after having informed Tiberius by letter that his son Drusus had been poisoned by Sejanus and Livia, killed herself. This disclosure brought about more executions. It is said that Tiberius would have pardoned Livia, but that her mother Antonia would not pardon her, and compelled her to die by starvation. The property of Sejanus was taken from the aerarium into the fiscus. (Tac. Ann. 6.2.)

In addition to the Annals of Tacitus, the chief authorities for the history of Sejanus are Suetonius, Tiberius, and Dio Cassius, lvii. Iviii. All the authorities are referred to by Tillemont, Histoire des Empereurs, vol. i. Velleius Paterculus is a panegyrist of Sejanus; and if Tacitus has told the truth of Sejanus, Paterculus was a vile flatterer. The fact that he dedicated his work to M. Vinicius, who was consul A. D. 30, shows the latest period at which he was writing. He may have perished with Sejanus. [PATERCULUS.]