Arthur Greenwood CH (8 February 1880 - 9 June 1954) was a prominent member of the Labour Party from the 1920s until the late 1940s. He rose to prominence within the party as secretary of its research department from 1920 and served as Parliamentary Secretary to the Ministry of Health in the short-lived Labour government of 1924.

Greenwood became deputy leader of the Labour Party under Clement Attlee, with Winston Churchill appointing him to the British War Cabinet as Minister without Portfolio in 1940. He was generally seen as ineffectual, but in May 1940 he emerged as Churchill's strongest and most vocal supporter in the lengthy War Cabinet debates on whether to accept or reject a peace offer from Germany. After that his position declined and he resigned in 1943. The same year, he was elected as Treasurer of the Labour Party, beating Herbert Morrison in a close contest.

Until the end of World War II, Greenwood also performed the function of Leader of the Opposition, though he did not receive the salary.

During the Attlee government, he served successively as Lord Privy Seal and Paymaster-General.

Greenwood's son Anthony Greenwood (later Lord Greenwood) (1911-1982) was an MP from 1946 and a member of Harold Wilson's governments.