Peninnis Head is the southernmost point of St Mary's, Isles of Scilly. The headland is within the Isles of Scilly Area of Outstanding Natural Beauty and part of the Heritage Coast. It is also a Geological Conservation Review site for its Quaternary geomorphology and was first designated a Site of Special Scientific Interest (SSSI) in 1971 for both its biological and geological interests. All of the land designated as Peninnis Head SSSI is owned by the Duchy of Cornwall. On the tip of the headland is a squat lighthouse built in 1911 by Trinity House as a replacement for the 17th century lighthouse on St Agnes.