Thomas Oldham (4 May 1816, Dublin – 17 July 1878, Rugby) was an Anglo-Irish geologist. He was educated at Trinity College, Dublin and studied civil engineering at the University of Edinburgh as well as geology under Robert Jameson. In 1838 he joined the ordnance survey in Ireland as a chief assistant under Joseph Ellison Portlock who was studying the geology of Londonderry and neighbourhood. Portlock wrote of him whenever I have required his aid … I have found him possessed of the highest intelligence and the most unbounded zeal