Merthen Manor (Cornish: Merdhin, meaning sea fort) is a 16th-century manor house in west Cornwall, England, UK. For most of its history it has been in the ownership of the prominent Cornish family, the Vyvyan family. The house is set in over 100 acres (40 ha) of woodland which, along with the shoreline of the Helford River, is designated as Merthen Woods Site of Special Scientific Interest (SSSI).