James William Lowther, 1st Viscount Ullswater, GCB, PC, JP, DL (1 April 1855 – 27 March 1949), was a British Conservative politician.

William Lowther, a grandson of William Lowther, 1st Earl of Lonsdale, and for 25 years Member of Parliament for Westmorland, and Alice, 3rd daughter of James Parke, 1st Baron Wensleydale, Lowther was educated at Eton College, King's College London where he took an Associateship degree, and at Trinity College, Cambridge, where he studied classics and law.

He was appointed 4th Charity Commissioner in 1887, and held junior ministerial office as Parliamentary Under-Secretary of State for Foreign Affairs from 1891 to 1892.

He was Chairman of the Speakers' Electoral Reform Conference in 1916–1917, of the Buckingham Palace Conference (on the partition of Ulster) in 1914, of the Boundary Commissions (Great Britain and Ireland) in 1917, of the Royal Commission on Proportional Representation in 1918, Devolution Conference in 1919, of the Royal Commission on London Government, 1921–1922; of Review Committee Political Honours, 1923–1924, and Statutory Commission on Cambridge University, 1923; of the Agricultural Wages Board from 1930 to 1940; of the Lords and Commons Committee on Electoral Reform, 1929–1930; and of BBC Enquiry Committee, 1935.

He was appointed to the Privy Council in 1898, created 1st Viscount Ullswater, of Campsea Ashe, in the County of Suffolk, on his retirement as Speaker in 1921,[2] and appointed a Knight Grand Cross of the Order of the Bath (GCB) in July 1921.