Mogontiacum (also Moguntiacum) is the Latin name of today's city of Mainz, which it bore during its almost 500 years as part of the Roman Empire. Mogontiacum had its origins in the legionary camp built by Drusus in 13/12 BCE, which was strategically located on a hill [de] above the Rhine and opposite the mouth of the Main on the Roman Rhine valley road [de].

The civilian settlements (vici) in the vicinity of the camp, which spread down the Rhine, quickly grew together to form a larger, urbanised settlement. However, unlike Colonia Claudia Ara Agrippinensium (Cologne) or Augusta Treverorum (Trier), Mogontiacum was primarily a military centre until the second half of the 4th century and was apparently not a colonia either. As a result, it never had the urban character of the other large Roman cities in Germany. Nevertheless, several monumental buildings were also erected here, as Mogontiacum was the provincial capital of the Roman province of Germania Superior with the seat of the governor as of the year 90 at the latest. After the middle of the 3rd century, when the Romans evacuated the Decumatian Fields, Mogontiacum once again became a border town and was devastated several times over the next 150 years by members of various Germanic peoples. After the end of the Roman period, but at the latest around 470, Mogontiacum belonged to the Frankish Kingdom after a brief transitional phase.

Some important remains of Mogontiacum have been preserved in the present-day city of Mainz, for example the Roman stage theatre, the Great Mainz Jupiter Column, the Drusus Stone and the Roman Stones [de], remains of the aqueduct of the legionary camp. The Roman-Germanic Central Museum, the Mainz State Museum and the Museum of Ancient Seafaring preserve numerous artefacts from Roman rule in Mainz.

The name Mogontiacum is composed of the Celtic name Mogo(n), the Celtic suffix -ontiu- (as in Vesontio/Besançon) and the affiliation suffix *-āko, Latinised to -(i)acum [de]. It thus contains the name of the Celtic god Mogon as a component. The name could have been derived from a Celtic settlement of the Aresaces, a section of the Treveri people, located in the immediate vicinity of the legionary camp. These were located at the end of the first century BCE in the area of today's Mainz-Weisenau and Mainz-Bretzenheim [de]. Mogontiacum was first mentioned in historiography by the Roman historian Tacitus in his work Histories, written at the beginning of the 2nd century, in connection with the Revolt of the Batavi. The derived spelling Moguntiacum is also common. Abbreviations and different spellings were also common at the time of Roman rule. For example, Moguntiacum was abbreviated to Moguntiaco in the Tabula Peutingeriana. Epigraphically, the city name is first documented on a milestone from the Claudian period.

The almost 500-year Roman history of Mogontiacum can be divided into four simplified periods: The first period begins with the founding of the city towards the end of the 1st century BCE and ends with the establishment of the province of Germania superior and the appointment of Mogontiacum as the provincial capital. The period between 90 CE and 260 CE covers the city's golden age until the fall of the Limes, when Mogontiacum once again became a border town of the Roman Empire. In the third period, from 260 to 350 CE, the city underwent profound changes in the face of internal turbulence in the Roman Empire and the growing threat from Germanic warriors. The final period from 350 CE to 470 CE reflects the decline of the city, which was plundered and devastated several times.

In the course of Augustus' expansionist endeavours starting in 16 BCE, his stepson Nero Claudius Drusus also advanced to the Middle Rhine and secured the area for the Roman Empire. By 13/12 BCE at the latest, possibly earlier, a double legionary camp was established on a hill above the Rhine and opposite the mouth of the Main. The military presence of the Romans at this location primarily secured their control over the Middle Rhine, the mouth of the Main and generally over the Main as one of the main routes into free Germania.

At the same time, another military camp was built just four kilometres south of today's Mainz district of Weisenau. This was mainly occupied by auxiliary troops, but was also used temporarily for the stationing of other legionaries. This was also the site of one of the late Latène Celtic settlements in the Mainz area. The local Celtic population belonged to the Aresaces, a section of the Gallic Treveri, who were located here in their settlement area furthest to the east.

Until the annexation plans were abandoned in 16 BCE, Mogontiacum served several times as a base for military operations as part of the Drusus campaigns (12 to 8 BCE), the campaign against the kingdom of Marbod (6 CE) and the Germanicus campaigns (14 to 16 CE) in Germania on the eastern side of the Rhine. Shortly afterwards, legionaries in Mogontiacum erected a cenotaph in the immediate vicinity of their camp to honor Drusus, who had died in 9 BCE. This monument is probably identical to the Drusus Stone on the Mainz citadel that still exists today. As early as the time of Drusus, a ship bridge was built above the mouth of the Main to cross the Rhine. In the first decade of the 1st century BCE, the bridgehead Castellum (Castellum Mattiacorum) on the right bank of the Rhine was founded and expanded, which became the nucleus of today's Mainz-Kastel (derived from the Latin castellum). The construction of a fixed wooden bridge (pile bridge) between Mogontiacum and Castellum can be dated to the year 27.