Archibald Henry Macdonald Sinclair, 1st Viscount Thurso (then Sir Archibald Sinclair) (October 22, 1890-June 15, 1970) was leader of the UK Liberal Party from 1935 until 1945.

Sinclair served on the Western Front during the First World War and rose to the rank of Major in the Guards Machine Gun Regiment. He served as second in command to Winston Churchill when Churchill commanded the 6th Battalion of the Royal Scots Fusiliers in the Ploegsteert Wood sector of the Western Front in 1915, Churchill having been disgraced after Gallipoli. They formed a lasting friendship that would become a significant political alliance in later decades.

In 1922 he entered the House of Commons as a Liberal MP for Caithness and Sutherland supporting David Lloyd George, defeating the incumbent Liberal supporter of Herbert Henry Asquith. He rose through the Liberal ranks as the party shrank in Parliament, becoming Chief Whip by 1930. In 1931 the Liberal Party joined the National Government of Ramsay MacDonald and Sinclair held the post of Secretary of State for Scotland. The following year he, together with other Liberal ministers, resigned from the government in protest at the Ottawa Convention introducing a series of tariff agreements. Sinclair and the Liberal leader, Sir Herbert Samuel, were thus the last Liberal politicians to sit in the Cabinet.

In the 1935 general election, Samuel lost his seat. Sinclair became the party's leader at the head of only twenty MPs. With the party now clearly marginalised as a third party on the fringe, with few distinct domestic policies, with a parliamentary party that was primarily a collection of individuals elected as much for themselves as for their party, and with the separate Liberal Nationals offering competition amongst Liberal inclined voters, Sinclair fought to make the Liberals once more a relevant force in British politics, taking up the issues of opposition to the continental dictatorships and working worked closely with Winston Churchill who was a backbencher at that time and generally shunned by his Conservative Party. When Churchill formed an all-party coalition government in 1940, Sinclair became Secretary of State for Air. However he did not sit in the small War Cabinet, though he was invited to attend meetings discussing any political matter. He remained a minister until May 1945 when the coalition ended. In the 1945 general election, he narrowly lost his seat. There was speculation that he might return to the Commons and the leadership, but he was unsuccessful in his attempt to re-enter Parliament in 1950. In 1952 he accepted elevation to the House of Lords as Viscount Thurso. He was expected to take up the leadership of the Liberal group in the House of Lords, but a series of strokes in the mid-fifties left him in a state of precarious health until his death in 1970.

In the 1990s, his grandson, John Thurso entered politics and now sits as MP for Caithness, Sutherland & Easter Ross.