William Francis Forbes-Sempill, 19th Lord Sempill, , (24 September 1893 – 30 December 1965) was a Scottish peer and record-breaking air pioneer, who was later shown to have 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.

In the 1920s, Sempill began giving military secrets to the Japanese. Although his activities were uncovered by British Intelligence, Sempill was not prosecuted for spying, and was allowed to continue in public life. He was eventually forced to retire from the Royal Navy in 1941, after being discovered passing on secret material to Tokyo shortly before Japan attacked Pearl Harbor and declared war on the United States and the British Empire.Sempill was known as "Master of Sempill" before succeeding his father to the titles of Lord Sempill and Baronet of Craigevar in 1934.