Sir Horace James Seymour GCMG CVO (26 February 1885 – 10 September 1978) was a British diplomat who served in Washington, D.C., Tehran, the Hague, Rome, and Chongqing. He was Principal Private Secretary to the British Foreign Secretary and Assistant Under-Secretary of State at the Foreign Office. His most senior appointment was as British Ambassador to China from 1942 to 1946.