The Basilica Julia was built on the site of the earlier Basilica Sempronia (circa 170 BCE) along the south side of the Forum, opposite the Basilica Aemilia. It was initially dedicated in 46 BCE by Julius Caesar, with building costs paid from the spoils of the Gallic War, and was completed by emperor Octavian Augustus, who named the building after his adoptive father. The ruins which have been excavated date to a reconstruction of the Basilica by the Emperor Diocletian, after a fire in 283 CE destroyed the earlier structure.

The Basilica is bordered on its short sides by two important ancient roads which led from the Tiber to the Forum: the Vicus Jugarius to the west, and the Vicus Tuscus to the east. A number of other important buildings surrounded the basilica on three sides, include the Temple of Saturn, Forum Square, Arch of Sepimius Severus, Temple of the Dioscuri (Castor and Pollux) and some warehouses and storehouses.

circa 54 BCE- 410 CE

Basilica of Sempronius
The basilica was built over the remains of two important Republican structures: the Basilica Sempronia, which was demolished by Caesar to make way for the new basilica, and pre-dating both, the house of Scipio Africanus, Rome's legendary general. The Basilica Sempronia was built in 169 BCE by Tiberius Sempronius Gracchus and required the demolition of the house of Africanus and a number of shops to make room.

First Basilica Julia
The first iteration of the Basilica Julia was dedicated in or around 54 BCE by Julius Caesar, however the construction most likely started in 46 BCE, though it was left to his heir Augustus to complete the construction and name it in honor of his adoptive father. The first Basilica Julia burned in 9 CE, shortly after completion, and was heavily reconstructed by emperor Augustus (Octavian).

Reconstruction by Augustus
The basilica reconstructed by emperor Augustus Octavian was extensively reconstructed, enlarged, and rededicated to his adoptive sons Gaius and Lucius in 12 CE.

Restoration by Septimius Severus and Diocletian
A significant fire occurred in Rome during the reign of Emperor Septimius Severus in 199 CE. This fire caused extensive damage to several important structures in the city, including the Basilica Julia, which had to be restored as a result. After the basilica was restored 199 CE fire by Septimius Severus it was subsequently reconstructed by the Emperor Diocletian after another fire in 283 CE.

Destruction During Visigothic Invasion and Subsequent Abandonment
The Basilica Julia was partially destroyed in 410 CE when the Visigoths sacked Rome, and the site slowly fell into ruin over the centuries. The marble was especially valuable in the medieval and early modern eras for burning into lime, a material used to make mortar. The remnants of kilns on the site, which were found in early excavations, confirmed that most of the building's components were destroyed in this way.

Conversion in to a Church
Part of the remains of the basilica were converted into a church, generally identified as that of Santa Maria de Cannapara, which is mentioned in catalogues from the 12th through the 15th centuries. Other parts of the basilica were sectioned off in the medieval period for the use of different trades. The marble workers, or marmorarii, took up most of the remaining space not occupied by the church in the 11th century for re-fashioning and selling marble architectural ornaments; the eastern aisle was occupied by the rope-makers and was called the Cannaparia as a result. In the 16th century, the long-buried site of the Basilica was used as a burial ground for patients of the adjacent Ospedale della Consolazione.

circa 54 BCE- 410 CE

Interior
The ground floor was divided into five east–west aisles inside, with the central aisle forming a large hall that measured 82x18 meters, sheltered by a three-story high roof. The adjoining aisles to the north and south of the central hall were divided by marble-faced brick columns which supported concrete arcades; the columns in turn supported the upper level of the basilica, which was used as a public gallery. The floor of the central hall was paved in colorful polychrome marble slabs, contrasting with the plain white marble of the adjoining aisles.

The building consists now only of a rectangular area, levelled off and raised about one metre above ground level, with jumbled blocks of stone lying within its area. A row of marble steps (inspect) runs full length along the side of the basilica facing the Via Sacra, and there is also access from a taller flight of steps (the ground being lower here) at the end of the basilica facing the Temple of Castor and Pollux.

circa 54 BCE- 410 CE

Facade
The Basilica's façade, as it appeared after the Augustan restoration, was two stories high and arcaded, with engaged Carrara marble columns decorating the piers between the arches on both levels.

circa 12 BCE- 410 CE

Tabernae (Shops)
The architectural value of the so-called tabernae, the first and the last of which are still partially filled with some tufa blocks, is now fully appreciated. The tabernae in question are four of the nineteen rooms along the south-western side of the Basilica Julia and facing onto a road (perhaps the vicus Unguentarius) that joined the Velabrum (the low valley in the city of Rome that connected the Forum with the Forum Boarium, and the Capitoline Hill with the western slope of the Palatine Hill). Built between 12 BCE and 12 CE in blocks of red tufa and travertine, the tabernae are evidence of the Augustan reconstruction of Basilica Julia. They were probably used as offices or as the headquarters of corporations, including that of the "money-changers" (nummulari de basilica lulia). Some of these rooms also had access to the upper floors through a complex system of stairs and galleries; the only clear traces of these can be seen in rooms II-III. The floors were in marble or opus spicatum (in the passageways and stairwells) whilst the ceilings were barrel-vaulted. The renovation of the Basilica after the fire in 283 CE particularly affected the upper sections of the supporting walls, rebuilt in brick, and the floors. In taberna IV the entrance staircase belongs to this renovation, whilst the presence of a "lime kiln" (a ditch where marble and travertine were turned into lime) marks the beginning of the burial of the Basilica Julia starting in the seventh century CE.

circa 1500 CE-

The earliest excavations of the Basilica Julia in the late 15th and 16th centuries CE were destructive, their main purpose being to recover valuable travertine and marble for re-use. In 1496, travertine was mined from the ruins to build the façade of the Palazzo Torlonia, the Roman palace of Cardinal Adriano Castellesi. There were also excavations in 1500, 1511–12, and 1514, as well as a destructive excavation in 1742 which uncovered the portion of the Cloaca Maxima which runs underneath the basilica. In the process, the Giallo antico yellow marble which covered the floor was stripped and sold to a stone-cutter.

The Chevalier Frédenheim also undertook excavations between November 1788 and March 1789; Frédenheim dismantled much of the remaining colored marble pavement and removed many architectural fragments. The site was excavated by Pietro Rosa in 1850, who reconstructed a single marble column and travertine supports. In 1852, segments of concrete vaulting with stuccowork coffering were unearthed but later destroyed in 1872.