The Sempill Mission was a British aeronaval technical mission led by Captain the Master of Sempill and sent to Japan in September 1921, with the objective of helping the Imperial Japanese Navy develop its aeronaval forces. The mission consisted in a group of 30 instructors and support staff, headed by Colonel Sempill, and stayed in Japan for 18 months. The Mission started training operation at the air base of Kasumigaura.