Nero Claudius Drusus, later Drusus Julius Caesar (his adoptive name) (13 BC-September 14, 23), was the only child of Roman Emperor Tiberius and his first wife, Vipsania Agrippina. He was born in 7 October 15 BC or 14 BC with the name Nero Claudius Drusus, and is also known to historians as Drusus II and Drusus Minor. Drusus was named after his paternal uncle the general Nero Claudius Drusus (who is sometimes called Nero Drusus, Drusus I, Drusus Major, or Drusus the Elder), who was Tiberius' younger brother. He was born and raised in Rome. Drusus was the first grandchild of statesman Marcus Vipsanius Agrippa and his first wife Caecilia Attica.

Despite his violent temper, Drusus showed promise with both military and politics. In 13, he was made a permanent member of the Senate committee Augustus had founded to draw up the Senate's daily business. However, because Drusus was only related to the Claudian side of the family, rather than both the Julians and Claudians, Augustus forced Tiberius to adopt Germanicus, who was married to Augustus's granddaughter, as his son and heir, removing Drusus from the succession. In 14, after the death of Augustus, Drusus suppressed a mutiny in Pannonia. In 15 he became a consul. He was also governor of Illyricum from 17 to 20. In 21 he was consul again, significantly with his father Tiberius as his colleague, while in 22 he received tribunicia potestas (tribunician power), a distinction reserved solely for the emperor or his immediate successor.

Drusus married his paternal cousin Livilla in 4. Their daughter Julia was born shortly after. They had twin sons Tiberius Gemellus and Tiberius Claudius Caesar Germanicus II Gemellus in 19, the latter of whom died still an infant in 23. In the same year, Germanicus died, making Drusus the new heir; Germanicus' wife Agrippina suspected Tiberius of having killed him to allow Drusus to become his heir, but this is unlikely.

Before the birth of the twins, Livilla may already have been in a relationship with Sejanus, Tiberius' Praetorian Prefect. Moreover Drusus, who was naturally irascible, had once in the course of a casual argument with Sejanus raised his fist and struck him in the face. By 23 it looked as if Drusus, who made no secret of his antipathy towards Sejanus, would succeed Tiberius as emperor. For reasons of self-survival, but also because he may have had designs on the supreme power, Sejanus needed to remove Drusus. Ancient sources (Tacitus, Suetonius, Cassius Dio) concur that with Livilla as his accomplice he poisoned her husband. If Drusus was indeed murdered, then it was done so skillfully that his death in 23 gave rise to no suspicion, having as he did a reputation for heavy drinking. Sejanus then (25) asked for Livilla’s hand in marriage but Tiberius forbade it.

Sejanus fell in 31 (October 18). A few days later (October 26) Sejanus' former wife Apicata committed suicide, but not before addressing a letter to Tiberius claiming that Drusus had been poisoned, with the complicity of Livilla. Drusus’ cupbearer Lygdus and Livilla's physician Eudemus were now tortured, and seemed to confirm Apicata’s accusation. By the end of the year Livilla too had perished, supposedly forcibly starved to death by her own mother, Antonia.

Drusus was an avid enthusiast of gladiator fights. In fact, we hear that the sharpest swords were named "Drusian" after him. Drusus is noted to have once come to blows with Sejanus in an argument. An earlier fight with a praetorian guard (possibly Sejanus as well) earned him the ironic nickname "Castor", after the patron god of the praetorians. He features under this name in the novel I, Claudius by Robert Graves, and in its BBC adaptation (in which he was played by Kevin McNally).

He is associated with the gourmand Apicius. Under Apicius' influence he disdained a certain vegetable of the cabbage family, earning a reprimand from Tiberius. Drusus is also recorded as using bitter almonds (five or six at a time) as a prophylactic against drunkenness.