Thomas Knox, 3rd Earl of Ranfurly (13 November 1816 – 20 May 1858), styled Viscount Northland between 1840 and 1858, was an Irish peer and Member of Parliament. He was the son of Thomas Knox, 2nd Earl of Ranfurly and his wife Mary Juliana Stuart, and represented Dungannon as a Member of Parliament between 9 June 1838 and 3 February 1851, when he resigned through the position of Steward of the Chiltern Hundreds. He was educated at St John's College, Cambridge.
He married Harriet Rimington, daughter of James Rimington, on 10 October 1848, and succeeded to the title of 4th Baron Welles of Dungannon, co. Tyrone on 21 March 1858, as well as the title of 3rd Earl of Ranfurly, the title of 3rd Baron Ranfurly of Ramphorlie, co. Renfrew, and the title of 4th Viscount Northland of Dungannon, co. Tyrone.
Children of Thomas Knox, 3rd Earl of Ranfurly and Harriet Rimington (26 November 1824 – 16 March 1891)
- Lady Agnes Henrietta Sarah Knox d. 29 Dec 1921
- Thomas Granville Henry Stuart Knox, 4th Earl of Ranfurly b. 28 Jul 1849, d. 10 May 1875
- Uchter John Mark Knox, 5th Earl of Ranfurly b. 14 Aug 1856, d. 1 Oct 1933 
- Hansard 1803–2005: contributions in Parliament by the Earl of Ranfurly