Trethowan is a Cornish surname, found both in Cornwall and amongst members of the Cornish diaspora. People with the name include:
Anthony Trethowan (1945–2015), Australian politician Arthur Trethowan (1863–1937), Australian politician Ian Trethowan (1922–1990), British television executive Illtyd Trethowan (1907–1993), British religious philosopher John Trethowan (born 1936), Australian rules footballer Pat Trethowan (born 1941), Australian rules footballer Rebecca Trethowan (born 1985), New Zealand rugby union player Perry Trethowan, a fictional detective in a series of books by Robert Barnard Sir William Trethowan ( ) British professor of psychiatry, Birmingham University
See also Trethowan's sign