Arthur Greenwood (8 February 1880 – 9 June 1954) was a British politician who was Deputy Leader of the Labour Party from 1935 to 1945. He had been Minister of Health in Ramsay MacDonald's government from June 1929 to August 1931. As his party's deputy leader in opposition, he was an outspoken critic of the government's appeasement policy. Following the Conduct of the War debate in May 1940, Greenwood and Labour leader Clement Attlee urged Neville Chamberlain to resign as Prime Minister. On their recommendation, Labour agreed to serve under his successor Winston Churchill who invited Attlee and Greenwood to join his select war cabinet, in which Greenwood became Churchill's main supporter. Greenwood had roving responsibilities as a minister without portfolio.
In February 1942, Greenwood resigned from the war cabinet to take over the necessary post of Leader of the Opposition. Although there was effectively no opposition to Churchill's government during the war, the position had to be filled to ensure the continued functionality of the House of Commons. Greenwood remained in situ until the end of the wartime coalition in May 1945, whereupon Attlee resumed the role as Labour Party leader. Polling in the general election took place on 5 July and, when the result was announced on the 26th, Labour had won a "landslide" victory.
Attlee became Prime Minister and Greenwood, having stepped down as deputy leader, was appointed Lord Privy Seal and Paymaster General. He resigned both roles in the spring of 1947, continuing to serve as Member of Parliament (MP) for Wakefield until his death in June 1954. His son Anthony (1911–1982) was the Labour MP for Rossendale from 1950 to 1970; he held cabinet positions in Harold Wilson's 1964–1970 government.