Sir Horace James Seymour GCMG CVO (26 February 1885 – 10 September 1978) was a British diplomat, Ambassador of the United Kingdom to China from 1942 to 1946.
The elder son of Hugh Francis Seymour and the great-grandson of Col. Hugh Henry John Seymour, Seymour was educated at Eton and Trinity College, Cambridge.
Seymour entered the Foreign Office and Diplomatic Service in 1908. He was Second Secretary at the British Embassy to the United States in 1919, First Secretary in the Netherlands in 1923 and in Italy in 1925, then from 1932 to 1936 Principal Private Secretary to the British Foreign Secretary. He was next British Minister ('Envoy Extraordinary and Minister Plenipotentiary') in Tehran, from 1936 to 1939, Assistant Under-Secretary of State at the Foreign Office, 1939 to 1942, and then British Ambassador to China, from 1942 to 1946. He retired in 1947.