Alfred Duff Cooper, 1st Viscount Norwich, GCMG, DSO, PC (22 February 1890 – 1 January 1954), known as Duff Cooper, was a British Conservative Party politician and diplomat who was also a military and political historian and writer.

First elected to Parliament in 1924, he lost his seat in 1929 but returned to Parliament in the 1931 Westminster St George's by-election, which was seen as a referendum on Stanley Baldwin's leadership of the Conservative Party. He later served in the Cabinet as Secretary of State for War and First Lord of the Admiralty. He resigned from the cabinet over the Munich agreement of 1938.

When Winston Churchill became prime minister in May 1940, he named Cooper as Minister of Information. In 1941, as a member of the Cabinet, he served as British Minister in Singapore before its fall to the Japanese. He later served an important role as representative to Charles de Gaulle's Free France (1943–44) and ambassador to France from 1944 to 1948.

Duff Cooper was born at Cavendish Square. He was the only son of society doctor Sir Alfred Cooper (1843–1908), a surgeon who specialised in the sexual diseases of the upper classes (his carriage was humorously known as "Cooper's Clap Trap") and Lady Agnes Duff, daughter of James Duff, 5th Earl Fife and descendant of King William IV. She had already eloped with two husbands, the first of whom she deserted and the second of whom died, before marrying Cooper in 1882. Duff Cooper had three older sisters and one older half sister from his mother's first marriage. He attended two prep schools, including Wixenford. He was unhappy at prep school, but was then very happy at Eton College.

At New College Oxford (1908–11), Cooper's Eton friendship with John Neville Manners won him entry into a circle of young aristocrats and intellectuals known as "The Coterie", including Patrick Shaw-Stewart, Raymond Asquith, Sir Denis Anson, Edward Horner and Lady Diana Manners. Cooper cultivated a reputation for eloquence and fast living and, although he had established a reputation as a poet, he earned an even stronger reputation for gambling, womanising and drinking in his studied emulation of the life of the 18th- and 19th-century Whig statesman Charles James Fox. Cooper's memory and gift for writing enabled him to do reasonably well at exams. He obtained a second in Modern History.

Following Oxford, Cooper entered the Foreign Service in October 1913, at the third attempt. During the war he worked in the commercial and the contraband departments. Owing to the national importance of his work at the cipher desk, he was exempted from military service until June 1917, when he joined the Grenadier Guards. He had not actively sought to join the army but was happy to be "released" as a result of the manpower shortage, as he thought joining the Army the decent thing to do. To his surprise, most of his fellow officer cadets were working-class and lower-middle-class men, almost all of whom had already served in the ranks.

Cooper spent six months on the Western Front, during which, Philip Ziegler writes, he proved himself "exceptionally courageous, resourceful, and a natural leader of men", at a time when the life expectancy of junior officers was very brief. He suffered a minor wound in the advance to the Albert Canal in August 1918, and was awarded the Distinguished Service Order (DSO) for conspicuous gallantry, a rare decoration for a junior officer. The citation for his DSO appeared in The London Gazette in November 1918 and reads as follows:

For conspicuous gallantry during an attack. Although the remainder of his company lost direction he led his platoon on to the objective and captured part of it. When supports arrived he led two sections against a machine-gun post, the four men immediately behind him were shot, but he went on alone and compelled the surrender of eighteen men and two machine guns. Later, with a patrol of six men he succeeded in capturing eighty-nine prisoners. He showed splendid courage and devotion to duty.