Marcus Agrippa Postumus (12 BC – August 14 AD), later named Agrippa Julius Caesar, was a grandson of Roman Emperor Augustus. He was the youngest child of Marcus Vipsanius Agrippa and Julia the Elder. Augustus initially considered Postumus as a potential successor and formally adopted him as his heir, before banishing Postumus from Rome in AD 6 on account of his ferocia ("beastly nature"). In effect, though not in law, the action cancelled his adoption and virtually assured Tiberius' emplacement as Augustus' sole heir. Postumus was ultimately executed by his own guards shortly after Augustus' death in AD 14.

Postumus was a member of the Julio-Claudian dynasty, the first imperial family of the Roman Empire. His maternal grandparents were Augustus and his second wife, Scribonia. Postumus was also a maternal uncle of Emperor Caligula, who was the son of Postumus' sister Agrippina the Elder, as well as a great-uncle of Nero, the last Julio-Claudian emperor, whose mother, Agrippina the Younger, was Caligula's sister.

Postumus was initially named "Marcus Agrippa" in honour of his father, who died shortly before his birth and so the surname Postumus was added. After the death of his older brothers, Lucius and Gaius Caesar, Postumus was adopted by his maternal grandfather, Augustus. A lex curiata ratified his adoption from which Postumus assumed the filiation Augusti f., meaning "son of Augustus". Postumus was then legally the son of Augustus, as well as his biological grandson. As a consequence, Postumus was adopted into the Julia gens and took the name "Julius Caesar". His name was changed to Agrippa Julius Caesar.

Agrippa Postumus was born in Rome in 12 BC, approximately three months after his father, Marcus Vipsanius Agrippa, died in the summer. He was born into the equestrian gens Vipsania. His father was one of Augustus' leading generals, and his mother, Julia the Elder, was the daughter of Augustus and his second wife, Scribonia. Postumus was the third son and last child of Agrippa and Julia; his older siblings were Gaius Caesar, Julia the Younger, Lucius Caesar and Agrippina the Elder. Both of his brothers, Gaius and Lucius, were adopted by Augustus after the birth of Lucius in 17 BC.

Before Gaius left Rome for Asia, Gaius and Lucius had been given the authority to consecrate the Temple of Mars Ultor (1 August 2 BC), and they managed the games that were held to celebrate the Temple's dedication. Postumus, still a student, participated in the Lusus Troiae ("Trojan Games") with the rest of the equestrian youth. At these games, according to Cassius Dio, 260 lions were slaughtered in the Circus Maximus, there was gladiatorial combat and a naval battle between the "Persians" and the "Athenians" and 36 crocodiles were slaughtered in the Circus Flaminius.

At first, Augustus opted not to adopt Postumus so that Agrippa would have at least one son to carry on his family name. However, the untimely deaths of principes Lucius (d. AD 2) and Gaius (d. AD 4) forced Augustus to adopt Postumus, his only remaining biological grandson, and Tiberius, Augustus' eldest stepson from his third wife, Livia, on 26 June AD 4 to secure the succession. He agreed to adopt Tiberius if Tiberius first adopted Germanicus. Upon his adoption into the Julii Caesares, Postumus assumed the name "Marcus Julius Caesar Agrippa Postumus". After the adoptions of AD 4, in the event of Augustus' death, the title of princeps would pass first to Tiberius and then to Germanicus.

It was not intended that Postumus receive the emperorship; instead, he was meant to be the heir to Augustus' bloodline. He would receive Augustus' name, property, and bloodline but not the title of princeps. Indeed, Postumus was not given any special schooling or treatment after his adoption. In AD 5, at the age of 17, he received the toga virilis, and his name was added to the list of aristocratic youth eligible for training as military officers. That differed greatly from the honours received by his brothers, both of whom were inducted into the Roman Forum by Augustus himself to commemorate their adoptions, given the title Princeps Iuventutis ("Leader of the Youth") and promised the consulship five years in advance, to be held when they reached 19.

In AD 6, an uprising began in the Roman province of Illyricum. Augustus sent Tiberius to crush the revolt with his army, and after a year of delayed results, he sent Germanicus in his capacity as quaestor to assist in bringing the war to a swift end. The reason, Dio says, that Germanicus was chosen over Postumus is because Postumus was of an "illiberal nature".