Iullus Antonius (43–2 BC) was a Roman magnate and poet. A son of Mark Antony and Fulvia, he was spared by the emperor Augustus after the civil wars of the Republic, and was married to the emperor's niece. He was later condemned as one of the lovers of Augustus's daughter, Julia, and killed himself.

Born in Rome, and named after his father's benefactor, Iullus and his elder brother had a disruptive childhood. His mother Fulvia gained many enemies including Octavian (nephew and adopted son of Julius Caesar). His half-sister, Claudia, had been Octavian's first wife; however, in 41 BC, Octavian divorced Claudia without having consummated the marriage and married Scribonia, the mother of Julia the Elder, Octavian's only child. Fulvia saw this as an insult on her family and, together with Iullus' uncle Lucius Antonius, they raised eight legions in Italy to fight for Antonius' rights against Octavian. The army occupied Rome for a short time, but eventually retreated to Perusia (modern Perugia). Octavian besieged Fulvia and Lucius in the winter of 41-40 BC, starving them into surrender. Fulvia was exiled to Sicyon, where she died of a sudden illness.

In the same year of Fulvia's death, Antonius' father Mark Antony married Octavian's full sister, Octavia Minor. The marriage had to be approved by the Senate as Octavia was pregnant with her first husband's child at the time. The marriage was for political purposes to cement an alliance between Octavian and Mark Antony. Octavia appears to have been a loyal and faithful wife who was good and treated her husband's children with the same kindness as her own. Between 40 BC–36 BC, Octavia lived with him in his Athenian mansion. She raised both of Mark Antony's sons and her children by her first husband together for the years of her marriage to their father. They all traveled with him to various provinces. During the marriage Octavia produced two daughters, Iullus' half-sisters Antonia Major and Antonia Minor. Antonia Major was the paternal grandmother of the Emperor Nero and maternal grandmother of the Empress Valeria Messalina. Antonia Minor was the sister-in-law of the Emperor Tiberius, paternal grandmother of the Emperor Caligula and Empress Agrippina the Younger, mother of the Emperor Claudius, and maternal great-grandmother/paternal great-aunt of the Emperor Nero.

In 36 BC, Mark Antony abandoned Octavia and her children in Rome and sailed to Alexandria to rejoin his former lover Cleopatra VII (they had already met in 41 BC and were parents of twin children). Mark Antony divorced Octavia circa 32 BC. Iullus and his half-sisters returned to Rome with Octavia while Iullus' elder brother, Marcus Antonius Antyllus, remained with his father in Egypt. Antyllus was raised by Cleopatra beside his father's children by her, Ptolemy Philadelphus, Alexander Helios and Cleopatra Selene II, and Antyllus' stepbrother Caesarion.

In the Battle of Actium, the fleets of Antony and Cleopatra were destroyed, and they fled to Egypt. In August 30 BC, Octavian, assisted by Marcus Vipsanius Agrippa, invaded Egypt. With no other refuge to escape to, Mark Antony killed himself by falling on his sword, having been tricked into thinking that Cleopatra had already done so. A few days later, Cleopatra did actually die by suicide.

Octavian and his army seized control of Egypt and claimed it as part of the Roman Empire. While Iullus' elder brother Antyllus and his stepbrother Caesarion were murdered by Octavian, he showed some mercy to their half-siblings Alexander Helios, Cleopatra Selene II and Ptolemy Philadelphus. They were given to Iullus' first stepmother Octavia to be raised as Roman citizens. In 27 BC, they returned to Rome, and Octavian was given the title of Augustus.

Following the civil wars, Iullus was granted high favours from Augustus, through Octavia's influence. In 21 BC Augustus wanted his daughter Julia the Elder to marry Agrippa, who at the time was married to Iullus' stepsister Claudia Marcella Major. Agrippa agreed to the marriage and so divorced Marcella. Marcella consequently obliged Iullus to marry her. Iullus and Marcella's children were the sons Iullus Antonius, Lucius Antonius and a daughter Iulla Antonia.

Iullus became praetor in 13 BC, consul in 10 BC, and Asian proconsul in 7/6 BC, and was highly regarded by Augustus. Horace refers to him in a poem, speaking of an occasion when Iullus intended to write a higher kind of poetry praising Augustus for his success in Gaul. Iullus was also a poet and is credited with having written twelve volumes of poetry on Diomedia some time before 13 BC, which has not survived.