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.

Monmouthshire is a county in the south east of Wales. It borders Powys to the north; the English counties of Herefordshire and Gloucestershire to the north and east; the Severn Estuary to the south, and Torfaen, Newport and Blaenau Gwent to the west. The largest town is Abergavenny, and the administrative centre is Usk.