Myarth is a hill in the Usk Valley in the county of Powys in South Wales, about 2 miles east of Crickhowell. Its summit at 292 metres (958 ft) is covered by trees, whilst the larger part of its slopes are also wooded. It is listed as a Marilyn. Myarth has an elongate form commonly ascribed to erosion by the west-to-east movement of the Usk Valley glacier during successive ice ages. The River Usk runs along the foot of the hill on its southern side.