William Francis Forbes-Sempill, 19th Lord Sempill AFC, AFRAeS, (24 September 1893 – 30 December 1965) was a British peer and record-breaking air pioneer who was later shown to have been a traitor who passed secret information to the Imperial Japanese military before the Second World War. Educated at Eton, he began his career as a pilot in the Royal Flying Corps and then served in the Royal Naval Air Service and Royal Air Force during the First World War. In 1921 Sempill led an official military mission to Japan that showcased the latest British aircraft. In subsequent years he continued to aid the Imperial Japanese Navy in developing its Navy Air Service.