Thomas Thomson FRSE FSA Scot (10 November 1768, Dailly, Ayrshire - 2 October 1852, Edinburgh) was a Scottish advocate, antiquarian and archivist who served as Principal Clerk of Session (1828-) and as secretary of the literary section of the Royal Society of Edinburgh (1812–20).
The eldest son of Thomas Thomson, who was minister of Dailly in Ayrshire, by his second wife, Mary, daughter of Francis Hay, he was born on 10 November 1768; John Thomson of Duddingston was a younger brother. After attending the parish school of Dailly, he entered the University of Glasgow at age 13, where he graduated M.A. on 27 April 1789. He then for two years attended classes in theology and law; and went to Edinburgh, where he was admitted advocate on 10 December 1793. He became a close friend of Walter Scott.
Thomson acquired a practice at the bar, particularly in cases demanding legal learning. Legal and historical antiquities later absorbed his attention. His main role was deputy clerk-register of Scotland, a new post to which he was appointed on 30 June 1806. His work mainly consisted of reforming the system of public registries and the method of the custody of records, in rendering these records accessible to research, in rescuing and repairing old records, and in editing the acts of the Scottish parliament and other governmental records under the authority of the Record Commission.
In February 1828 Thomson was chosen one of the principal clerks of the court of session. On the institution of the Bannatyne Club in 1823 he had been chosen vice-president, and on the death of Scott in 1832 he succeeded as president. Thomson, however, was lax on finance. After an inquiry into the accounts of the register office in 1839 he was removed from the office of deputy clerk-register. At this time he was living at 127 George Street in Edinburgh.