William Stephen Ian Whitelaw, 1st Viscount Whitelaw (28 June 1918 – 1 July 1999) was a British Conservative Party politician who served in a wide number of Cabinet positions, most notably as Home Secretary from 1979 to 1983 and as de facto Deputy Prime Minister of the United Kingdom from 1979 to 1988. He was Deputy Leader of the Conservative Party from 1975 to 1991.

After the Conservative Party won an unexpected victory at the 1970 general election, Whitelaw was appointed as Leader of the House of Commons and Lord President of the Council by Prime Minister Edward Heath. After the suspension of the Stormont Parliament resulted in the imposition of direct rule, Whitelaw served as Secretary of State for Northern Ireland from 1972 to 1973. He also served under Heath as Secretary of State for Employment from 1973 to 1974 and as Chairman of the Conservative Party from 1974 to 1975.

Whitelaw served Prime Minister Margaret Thatcher throughout her leadership of the Conservative Party as deputy party leader. He served as de facto Deputy Prime Minister between 1979 and 1988 and as Home Secretary from 1979 to 1983. He stepped down as a Member of Parliament at the 1983 general election, and was appointed as a Member of the House of Lords. He served as Leader of the House of Lords and Lord President of the Council from 1983 to 1988. He was a captain of The Royal and Ancient Golf Club of St Andrews.

Whitelaw was born in Edinburgh, Scotland, and raised at the family home, "Monklands", on Thurlow Road in Nairn. He never met his father, William Alexander Whitelaw, born 1892, a member of a Scottish family of the landed gentry, who died in 1919 after service in the Argyll and Sutherland Highlanders in the First World War, when his son was still a baby. Whitelaw was brought up by his mother, Helen, a daughter of Major General Francis Russell, MP for Cheltenham and a military attaché, and his paternal grandfather William Whitelaw. One of his great-aunts by marriage, born Dorothy Sarah Disraeli, was the niece of former Prime Minister Benjamin Disraeli.

Whitelaw was educated first at Wixenford School, Wokingham, before passing the entrance exam to Winchester College. From there he went up to Trinity College, Cambridge, where he won a blue for golf and joined the Officer Training Corps. By chance he was in a summer camp in 1939 on the outbreak of the Second World War and was granted a regular, not wartime, commission in the British Army, in the Scots Guards, later serving in the 6th Guards Tank Brigade, a separate unit from the Guards Armoured Division. He commanded Churchill tanks in Normandy during the Second World War and during Operation Bluecoat in late July 1944. His was the first Allied unit to encounter German Jagdpanther tank destroyers, being attacked by three out of the twelve Jagdpanthers which were in Normandy.

The battalion's second-in-command was killed when his tank was hit in front of Whitelaw's eyes; Whitelaw succeeded to this position, holding it, with the rank of major, throughout the advance through the Netherlands into Germany and until the end of the war. He was awarded the Military Cross for his actions at Caumont; a photograph of Field Marshal Bernard Montgomery pinning the medal to his chest appears in his memoirs. After the end of the war in Europe, Whitelaw's unit was to have taken part in the invasion of Japan, but the Pacific War ended before this. Instead he was posted to Palestine, before leaving the army in 1946 to take care of the family estates of Gartshore and Woodhall in Lanarkshire, which he inherited on the death of his grandfather.

Following early defeats as a candidate for the constituency of East Dunbartonshire in 1950 and 1951, Whitelaw was elected as Member of Parliament (MP) for Penrith and the Border at the 1955 general election and represented that constituency for 28 years. He held his first government posts under Harold Macmillan as a Lord of the Treasury (government whip) between 1961 and 1962 and then under Macmillan and Alec Douglas-Home was Parliamentary Secretary to the Ministry of Labour between 1962 and 1964. After the Conservatives lost the 1964 election, Douglas-Home appointed Whitelaw as Opposition Chief Whip. He was sworn of the Privy Council in January 1967.

When the Conservatives returned to power in 1970 under Edward Heath, Whitelaw was made Lord President of the Council and Leader of the House of Commons, with a seat in the cabinet. Upon the imposition of direct rule in March 1972, he became the first Secretary of State for Northern Ireland, serving in that capacity until November 1973. During his time in Northern Ireland he introduced Special Category Status for paramilitary prisoners. He attempted to negotiate with the Provisional Irish Republican Army, meeting its Chief of Staff Seán Mac Stiofáin in July 1972. The talks ended in an agreement to change from a seven-day truce to an open-ended truce; however, this did not last long. As a briefing for prime minister Heath later noted, Whitelaw "found the experience of meeting and talking to Mr Mac Stíofáin very unpleasant". Mac Stiofáin in his memoir complimented Whitelaw, saying he was the only Englishman ever to pronounce his name in Irish correctly.