51 Nemausa ( /nɨˈmɔːzə/) is a large asteroid-belt asteroid similar to 1 Ceres in composition. It was discovered by one "A. Laurent", an obscure figure about whom little is known. Laurent made the discovery from the private observatory of Benjamin Valz in Nîmes, France. The house, at 32 rue Nationale in Nîmes, has a plaque commemorating the discovery. With Laurent's permission, Valz named the asteroid after the Celtic god Nemausus, the patron god of Nîmes during Roman times.
Nemausa measures 147.9km in diameter. Lightcurve data suggests that it may have a small moon. It may have a water content of about 14%. Nemausa has been studied by radar.