Lassus Castle, Medieval castle on the Ourthe River in Hamoir, Belgium.

Lassus Castle stands on the banks of the Ourthe about 1.5 km upstream from Hamoir. Its oldest part is a small early 14th-century keep with an adjoining turret, once crenellated and now roofed. The building was entirely remodelled in the 18th century in Louis XIV style, whose most visible feature is the pediment facing the river. The river frontage and the keep are the least altered parts.

The oldest part is a keep with a small adjoining tower, built in the early 14th century and once crenellated. The castle long belonged to the de Maillen family, hereditary mayors of Hamoir, who added a small castle chapel about 100 metres away in 1633. In the 18th century the building was entirely remodelled in the Louis XIV style, and in 1897 it passed to Georges Chaudoir, a wealthy Liège banker who added a large tower and nearly doubled the volume of the buildings.

The castle's small chapel was set up in 1633 by the de Maillen family, about a hundred metres from the building. Opposite the castle stands a chapel dedicated to Saint Peter, founded in 1396, whose Renaissance interior holds tombstones of the de Donéa and de Maillen families.

The castle stands on the banks of the Ourthe at Lassus, about 1.5 km upstream from Hamoir. The river frontage and the medieval donjon are the parts least altered by later work, so they are the features to look for when approaching from the water or the road. The former stables and barns still half-close a courtyard.

The chapel opposite the castle, dedicated to Saint Peter, was founded in 1396, older than the castle's 1633 internal chapel. Its Renaissance interior holds tombstones of the de Donéa and de Maillen families, marking the families' long ties to the site.