Octavia the Younger (Latin: Octavia Minor ; 69 BC – 11 BC) was the elder sister of the first Roman emperor, Augustus (known also as Octavian), the half-sister of Octavia the Elder, and the fourth wife of Mark Antony. She was also the great-grandmother of the Emperor Caligula and Empress Agrippina the Younger, maternal grandmother of the Emperor Claudius, and paternal great-grandmother and maternal great-great-grandmother of the Emperor Nero.

One of the most prominent women in Roman history, Octavia was respected and admired by contemporaries for her loyalty, nobility and humanity, as well as for maintaining traditional Roman feminine virtues.

Octavia was born around 69 BC. Full sister to Augustus, Octavia was the only daughter born of Gaius Octavius' second marriage to Atia, niece of Julius Caesar. Octavia was born in Nola, present-day Italy; her father, a Roman governor and senator, died in 59 BC from natural causes. Her mother later remarried, to the consul Lucius Marcius Philippus. Octavia spent much of her childhood travelling with her parents. Marcius was in charge of educating Octavia and her brother Octavian, later known as Augustus.

Some time before 54 BC, her stepfather arranged for her to marry Gaius Claudius Marcellus. He was a member of the influential plebeian branch of the Claudian family and descended from Marcus Claudius Marcellus, a famous general in the Second Punic War. In 54 BC, Octavia's great-uncle Julius Caesar is said to have been anxious for her to divorce her husband so that she could marry Pompey, who had just lost his wife Julia (Caesar's daughter, and thus Octavia's cousin once removed). The couple did not want to get a divorce, so instead Pompey declined the proposal and married Cornelia Metella. Thus, Octavia's husband continued to oppose Julius Caesar, including in the crucial year of his consulship, 50 BC. Civil war broke out when Caesar invaded Italy from Gaul in 49 BC.

Marcellus, a friend of Cicero, was an initial opponent of Julius Caesar when Caesar invaded Italy, but did not take up arms against his wife's great uncle at the Battle of Pharsalus, and was eventually pardoned by him. In 47 BC he was able to intercede with Caesar for his cousin and namesake, also a former consul, then living in exile. Presumably, Octavia continued to live with her husband from the time of their marriage (she would have been between 12 and 15 when they married) to her husband's death. They had three surviving children: Claudia Marcella Major, Claudia Marcella Minor and Marcus Claudius Marcellus. All three were born in Italy. However, according to the anonymous Περὶ τοῦ καισαρείου γένους Octavia bore Marcellus four sons and four daughters. Her husband Marcellus died in May 40 BC.

By a Senatorial decree, Octavia married Mark Antony in October 40 BC, as his fourth wife (his third wife Fulvia having died shortly before). This marriage had to be approved by the Senate, as she was pregnant with her first husband's child, and was a politically motivated attempt to cement the uneasy alliance between her brother Octavian and Mark Antony; however, Octavia does appear to have been a loyal and faithful wife to Antony. Between 40 and 36 BC, she travelled with Antony to various provinces and lived with him in his Athenian mansion. There she raised her children by Marcellus as well as Antony's two sons; Antyllus and Iullus, as well as the two daughters of her marriage to Antony, Antonia Major and Antonia Minor who were born there. During this period, she became the first (or second) Roman woman to have coins minted bearing her image; only Antony's previous wife Fulvia may have pre-empted her.

The alliance was severely tested by Antony's abandonment of Octavia and their children in favor of his former lover Queen Cleopatra VII of Egypt (Antony and Cleopatra had met in 41 BC, an interaction that resulted in Cleopatra bearing twins, Alexander Helios, a boy, and Cleopatra Selene, a girl). After 36 BC, Octavia returned to Rome with the daughters of her second marriage. On several occasions she acted as a political advisor and negotiator between her husband and brother. For example, in the spring of 37 BC, while pregnant with her daughter Antonia Minor, she was considered essential to an arms deal held at Tarentum, in which Antony and Augustus agreed to aid each other in their Parthian and Sicilian campaigns. She had won over her brother's advisers Agrippa and Maecenas and convinced him to renew their alliance. She was hailed as a "marvel of womankind". In 35 BC, after Antony suffered a disastrous campaign in Parthia, she brought fresh troops, provisions, and funds to Athens. There Antony had left a letter for her, instructing her to go no further. Mark Antony divorced Octavia in late 33 BC. In 33, Antony sent men to eject her from his house in Rome. She became sole caretaker of their children, except for Antyllus who was already with his father in the East. After Antony's suicide in 30 BC, her brother executed Antyllus but allowed Octavia to raise Antony's younger son Iullus by Fulvia as well as his children by Cleopatra (the two sons Alexander Helios and Ptolemy Philadelphus, and one daughter, Cleopatra Selene II).

In 35 BC, Augustus accorded a number of honours and privileges to Octavia, and also to his wife, Livia – previously unheard of for women in Rome. They were granted sacrosanctitas, meaning it was illegal to verbally insult them. Previously, this had only been granted to tribunes. Livia and Octavia were made immune from tutela, the male guardianship which all women in Rome except for the Vestal Virgins were required to have. This meant they could freely manage their own finances. Finally, they were the first women in Rome to have statues and portraits displayed en masse in public places. Previously, only one woman, Cornelia, mother of the Gracchi, had been part of the public statues displayed in Rome. In Augustus' rebuilding of Rome as a city of marble, Octavia was featured. In all her representations, she wore the "nodus" hairstyle, which at the time was considered conservative and dignified, and worn by women from many classes.