Grosmont Castle is a ruined castle in the village of Grosmont, Monmouthshire, Wales. The fortification was established by the Normans in the wake of the invasion of England in 1066, to protect the route from Wales to Hereford. Possibly commissioned by William fitz Osbern, the Earl of Hereford, it was originally an earthwork design with timber defences. In 1135, a major Welsh revolt took place, and in response King Stephen brought together Grosmont Castle and its sister fortifications of Skenfrith and White Castle to form a lordship known as the "Three Castles", which continued to play a role in defending the region from Welsh attack for several centuries.

Grosmont is a village and community near Abergavenny in Monmouthshire, Wales. The population taken at the 2011 census was 920. The wider community (parish) includes the villages of Llangattock Lingoed, Llangua and Llanvetherine.