Philip Rashleigh III (1729–1811) of Menabilly, Cornwall, was an antiquary and Fellow of the Royal Society and a Cornish squire. He is probably most notable for collecting from the labourers who found it, and publishing, the Trewhiddle Hoard of Anglo-Saxon treasure, which still gives its name to the "Trewhiddle style" of 9th century decoration.
He was born at Aldermanbury, London, on 28 December 1729, the eldest son and heir of Jonathan Rashleigh III (1693–1764), of Menabilly, MP for Fowey in Cornwall, by his wife Mary Clayton, daughter of Sir William Clayton, 1st Baronet (died 1744) of Marden in Surrey.
He matriculated from New College, Oxford, 15 July 1749, and contributed to the poems of the university on the death of Frederick, Prince of Wales, a set of English verses, which is reprinted in Nichols's Select Collection of Poems (viii. 201–2); he left Oxford without taking a degree. At the death of his father he was elected member for the family borough of Fowey, on 21 January 1765, and sat continuously, in spite of contests and election petitions, until the dissolution of 1802, when he was known as the "Father of the House of Commons". His knowledge of Cornish mineralogy procured his election as a Fellow of the Society of Antiquaries and Fellow of the Royal Society in 1788.
A portrait of Rashleigh, seated in a chair, was painted by John Opie about 1795, and is now in the Royal Cornwall Museum, Truro. It is a "fine specimen of the painter's best period".