Trethewey is a surname of Cornish origin. It is derived from any of the various settlements in Cornwall called Trethewey.Hanks, Patrick; Hodges, Flavia; et al. (2002). The Oxford Names Companion. Oxford University Press. p. 621.
Notable people with the surname include:
Fred Trethewey (born 1949), British archdeacon Natasha Trethewey (born 1966), American poet Richard Trethewey (born c. 1955), American plumber and television personality Robert Trethewey (1902–1989), Australian politician Tom Trethewey (born 1944), American swimmer William Griffith Trethewey (1865–1926), Canadian inventor, prospector and farmer William Trethewey (1892–1956), New Zealand sculptor and monumental mason