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
He discovered radiating fans shaped impressions in the town of Bray in 1840. He showed this to the English palaeontologist Edward Forbes, who named it Oldhamia after him. Forbes declared them to be bryozoans, however later workers ascribed it to other plants and animals. For a while these were considered the oldest fossils in the world.
