Underdown is a surname. Notable people with the surname include:Charles Edward Underdown (1908–1989), English actor
David Underdown (1925–2009), English historian
Emanuel Maguire Underdown (1831–1913), English barrister, author and industrialist
Emily Underdown (1863–1947), English author
George Underdown (1859–1895), English cricketer
Harry Charles Bailee Underdown (1877–1963), English barrister and industrialist
James Underdown, executive director of The Center for Inquiry (CFI) West in Los Angeles
Thomas Underdown, English poet and translator