Derick Smith Thomson (5 August 1921 – 21 March 2012), known as Ruaraidh MacThòmais in his native Gaelic, was a Scottish poet, publisher, lexicographer, academic and writer. He was originally from Lewis, but spent much of his life in Glasgow, where he was Professor of Celtic at the University of Glasgow from 1963 to 1991. He is best known for setting up the publishing house Gairm, along with its magazine, which was the longest-running periodical ever to be written entirely in Gaelic, running for over fifty years under his editorship. Gairm has since ceased, and has been replaced by Gath. He was an Honorary President of the Scottish Poetry Library, and a Fellow of the Royal Society of Edinburgh and the British Academy. In June 2007, he received an honorary degree from Glasgow University.
Thomson is originally from Upper Bayble (Pabail Uarach) on Lewis, the same village that produced two other Gaelic writers of note, Iain Crichton Smith and Anne Frater. His parents, James Thomson and Christina Smith, were both primary school teachers.
Educated at the Nicolson Institute in Stornoway, he went on to the Universities of Aberdeen; Cambridge and Bangor University. He would later teach at Edinburgh, Glasgow and Aberdeen. He became Professor of Celtic at Glasgow in 1963, and retired in 1991. He was Chairman of the Gaelic Books Council 1968–91; President Scottish Gaelic Texts Society; former member of Scottish Arts Council and was the first recipient of the Ossian Prize in 1974. Professor Thomson was Chairman of the SNP's Gaelic Committee in the Seventies.