The gens Julia was one of the most prominent patrician families of ancient Rome. From the early decades of the Republic, members of this gens served in the highest offices of the Roman state, beginning with Gaius Julius Iulus, consul in 489 BC. However, the Julii are perhaps best known for Gaius Julius Caesar, the dictator and adoptive father of the emperor Augustus, through whom the name was passed to the Julio-Claudian dynasty of the first century AD. The nomen Julius became very common in imperial times, as the descendants of persons enrolled as citizens under the early emperors began to make their mark in history.

According to Roman tradition, the Julii were among the Alban families brought to Rome when their city was destroyed by Tullus Hostilius, the third Roman king, who enrolled them among the patricians and accorded them seats in the Roman Senate. Diodorus Siculus reported that the Julii had held the position of Rex Sacrorum at Alba Longa, the result of a compromise with the Silvii, who held the kingship. Some scholars have pointed to this as evidence of dual kingship in the earliest period of various cities of Latium, including Rome and Lanuvium, which also had the post of Rex Sacrorum.

A seemingly contradictory tradition places the Julii at Rome even earlier, following the death of Romulus. A certain Proculus Julius is reported to have witnessed Romulus descending from the heavens, bidding the Romans not weep for his loss, but to take up his worship as the god Quirinus. However, as this story concerns a miraculous event, and might have been influenced by the fame of the Julii in later times, it cannot be regarded as evidence of the period that the family first settled at Rome.

The Julii were also connected to Bovillae from an early period, some of them possibly having settled there after the fall of Alba Longa. An altar inscription in the theatre of Bovillae, dating from around the beginning of the first century BC, speaks of the Julii carrying out sacrifices according to the Alban rites. In imperial times the emperor Tiberius dedicated a sacrarium, or chapel, to the Julii at Bovillae, alongside a statue of Augustus.

In the later Republic, it was fashionable for aristocratic families to claim descent from the gods and heroes of Greek and Roman myth. The Julii claimed descent from Iulus, said to be the same person as Ascanius, the son of Aeneas, and founder of Alba Longa. In Greek myth, Aeneas was the son of Venus and the Trojan prince Anchises. The traditions upon which these claims were based were not always clear; the historian Livy was unsure whether to regard Iulus and Ascanius as the same person, or perhaps two brothers—one the son of Creüsa, Aeneas' first wife, lost in the sack of Troy—and the other the son of Lavinia, the daughter of Latinus, whom Aeneas married after landing in Italy. The late Roman grammarian Servius went to some effort to prove the identity of Ascanius and Iulus through etymology.

The importance attached to their mythic ancestry may have served a further purpose: after their initial prominence under the early Republic, the Julii sank into obscurity, and they are hardly mentioned for a century and a half beginning in the mid-fourth century BC. When at last they emerged and once again began to assume positions of importance, emphasizing their ostensible connections with Rome's foundation myths might have helped to restore their prestige. As he rose to prominence in the Roman state, Caesar regularly alluded to these myths, notably doing so when speaking at the funeral of his aunt Julia, and using Venus Genetrix as the watchword for his soldiers at Pharsalus and Munda. Coins of the Julii bear the likeness of Venus, and Roman writers willingly readily furthered a myth that served to glorify the emperors.

The Julii were amongst the most conservative patrician families in terms of praenomina. With only rare exceptions, they limited themselves to just three names: Lucius, Gaius, and Sextus, all of which were common throughout Roman history. The consular fasti supply two others, evidently used among the early Julii: Vopiscus was borne by the consul of 473 BC, and he in turn had a son, Spurius. Vopiscus was evidently an old praenomen that had fallen out of use, and is otherwise found only as a surname. Spurius was common enough during the early Republic, but was scarce in later periods, preserved chiefly by a few families in which it was traditional. Livy also gives Gnaeus as the praenomen of Gaius Julius Mento, consul in 431 BC.

Proculus, borne by the legendary figure who reportedly witnessed the apotheosis of Romulus, was another old praenomen that, though uncommon, was still in general use during the early Republic. Like Vopiscus, in later times it occurs as a cognomen. It is also possible that Iulus or Iullus, the name from which the Julii derived their gentilicium, was originally a praenomen. Perhaps with this reason in mind, Mark Antony, the friend and colleague of Caesar, and who was descended from the Julii, named one of his sons Iullus.