Novaesium was the name the Romans used for the successive legionary camps and fortress at what is now the city of Neuss, on the west bank of the Rhine, in Germany. The earliest occupations, dating from the late 1st century BC to the early 1st century AD, were a succession of earth and timber camps with the legionaries living in tents. In around AD 43, a large legionary fortress was begun, which was progressively fortified with stone walls, gates, and turrets, along with more permanent barracks, officers' quarters and administrative buildings. As the Romans abandoned an expectation of a continually expanding empire the fortress became a permanent structure, and helped to create the Limes, limits of the Roman Empire, along this stretch of the Rhine Valley. The fortress was made smaller in the early 2nd century but remained an auxiliary base which helped define and defend the north-eastern limits of the Roman Empire for a further 200 years.

The foundations of the stone fortress were discovered by Constantin Koenen in the late 19th century. When excavated it was the first complete ground plan of a legionary fortress and came to epitomise the 'playing card' style Claudian era fortress. Further excavations in the 1950s to 1980s revealed progressively more complex precursor camps to the west of Koenen's excavations, leaving a chronology and terminology which remains to some extent unresolved. The whole site was developed for housing as the excavations progressed, limiting the scope for subsequent discovery or clarification. In 2021 the lower Rhine fortifications were inscribed as the Lower Germanic Limes UNESCO World Heritage Site, a series of 102 locations from south of Bonn (Germany) to the North Sea coast (the Netherlands).

Novaesium is well attested within classical writings. Tacitus mentions the name in ten different passages of his Histories, describing troop movements, retreats, battles, defections and defeats during the turbulent year of 69 CE. Legio I Germanica had been stationed variously at Cologne, Novaesium and Bonn since 16 BC and had been caught up in the mutiny of 14 CE. Legio XVI Gallica were stationed in Novaesium from 43 to 70 CE. During 69 CE, the Year of the Four Emperors, in the turmoil following Nero's reign, parts of both legions had marched on Rome in support of Vitellius, and subsequently were both disbanded the following year after their inability to deal with the Revolt of the Batavi.

The Legio XX Valeria Victrix were at Novaesium until they joined Claudius's invasion of Britain in AD 43 and Legio VI Victrix was the legion brought into Novaesium to replace the two disbanded legions in 69/70 until they were relocated to Xanten sometime around the end of the first century when the legionary fortress was abandoned.

The Antonine Itinerary of around AD 200 gives a firmer basis for locating Novaesium at Neuss, and the map known as the Tabula Peutingeriana from around AD 300 meant its location was not in doubt. Ammianus Marcellinus's History written in 359, shows that as a settlement Novaesium retained its significance after the legions had left. The site of the fortress was re-used for a much smaller auxiliary fort, probably housing a cavalry unit (ala) for the next 200 years. The gravestone of a rider from the ala Afrorum veterana was found in Neuss and is dated around 100.

The 'Nivisium castellum' is also mentioned by Gregory of Tours. Around 575 he wrote Book II of his History of the Franks and section 9, quoting a now lost work by Sulpicius Alexander about events in 388, tells of how "Quintinus crossed the Rhine with his army near the stronghold of Neuss, and at his second camp from the river he found dwellings abandoned by their occupants and great villages deserted. For the Franks pretended to be afraid and retired into the more remote tracts, where they built an abattis on the edge of the woods." The Romans were subsequently trapped in the woods and marshes where "the ranks were thrown into disorder and the legions cut in pieces."

A variety of Roman artifacts had been found in the town through the 17th and 18th centuries, including two gravestones belonging to the XX and XVI Legions and one placed by a veteran of an auxiliary unit. In 1839 the first Neuss antiquities association was formed by a Prussian medical officer, Dr. Hermann Joseph Jaeger. In 1844 they carried out excavations at Reckburg, discovering a small Roman fort, and in 1845 the Neuss Municipal Museum was begun, to display the growing quantity of finds.

Fresh energy was injected in 1877 when a new generation of enthusiastic archaeologists founded the 'Vereins für Altertumskunde und Geschichte' (Association for Antiquities and History), whose members included Oskar Rautert and the 23 year old Constantin Koenen [de]. After further work at the Reckburg excavations, attention was turned to the location of the legionary fortress, which had been assumed to be beneath the old town centre. Koenen pursued a theory that it was to the south of the town, and in 1886 got permission from the Bonn Provincial Museum to make a test excavation on open ground towards the area where the Rhine is joined by the river Erft. The findings were positive, and over the next 13 years, Koenen worked to reveal the whole of the ground plan of the legionary fortress. Although there had been no above-ground indications, it was found that the foundation levels were well-preserved. The scale of the excavations grew to a take in ever more of the fortress. To reveal the 25 hectares (62 acres) site the excavation team had to remove some 50,000 cubic meters/yards of overlying soil using shovels and wheelbarrows. This was part of a new archaeological movement across Europe, and for the first time, a full ground plan of a Legionary fortress was known. When the final results were published in 1904 it was something of a sensation. Koenen's excavations had revealed a 'playing card' shaped stone-walled rectangular fortress with rounded corners, turrets at intervals and substantial gates on each of the 4 sides. It was dated to the period from the mid-first century, when Emporer Claudius was consolidating the border along the Rhine Valley, and then moving the legions north-west to enable the expansion of Rome across the sea to Britain. The fortress ground plan identified 62 infantry barrack blocks around a central area with a colonaded administration block, Governor's residence, officers' houses, cavalry barracks, workshops, stores, a bath-house, infirmary and all that was needful for a legion of some 5,000 soldiers.