Flexum, Roman military camp in Mosonmagyaróvár, Hungary

Flexum was a Roman fort on the Pannonian limes, guarding the Small Danube (Mosoni-Duna), a branch of the main river. Its exact position is unknown; the military centre is hypothetically placed in today's Lucsony district of Mosonmagyaróvár, while excavations there have documented the associated civilian settlement (vicus). The name Ad Flexum means "at the bend", referring to a meander of the Small Danube.

Ad Flexum was a Roman fort on the Pannonian limes, guarding the Small Danube (Mosoni-Duna), a branch that for long stretches formed the frontier. The fort's exact position is unknown; its civil settlement (vicus) is documented in today's Lucsony district of Mosonmagyaróvár. In 169–171 Germanic attacks from the left bank nearly destroyed the limes and the settlement; the 3rd century brought renewed prosperity, with a population of 3000–4000. After Valentinian I's death in 375, Hun attacks forced the inhabitants to flee, and later Lombard and Avar rule largely wiped out the site.

The Latin name Ad Flexum means "at the bend", referring to a meander of the Small Danube, the branch the fort guarded. The site is also recorded in the Itinerarium Antonini and the Tabula Peutingeriana, which places it 16 Roman miles from Gerulata. The name was first linked to Moson by the geographer Philipp Clüver (1580–1623).

The Roman remains lie beneath a medieval fortress and cannot be seen above ground, so visiting the Hansági Museum in town is the best way to get a sense of the site. The museum holds objects found during excavations here and gives good context for what once stood at the river confluence.

The medieval fortress built over the Roman fort was not placed there by chance: builders reused the same spot because the river bend offered the same natural advantage it had given Roman commanders centuries earlier. This means the ground beneath the current structure holds layers from two very different military eras.