Captain William Frederick Longstaff (1879–1953) was an Australian painter and war artist best known for his works commemorating those who died in the First World War.
Born in Ballarat, Victoria, Longstaff studied art privately before joining the military and serving in the Boer War as a member of the South African Light Horse. He was the cousin of portrait painter Sir John Longstaff.
Upon returning to Australia, Longstaff continued to paint and teach art. He enlisted in the Australian Imperial Force at the outbreak of the First World War and was injured in the Gallipoli campaign. In October 1915 he joined a remount unit and served in France and Egypt before being evacuated to England in 1917. In England, he began drawing again and was trained in the art of camouflage. During his time in Egypt, Longstaff had made images of the ANZAC Mounted Division and the other units. Upon his appointment as an Official War Artist in 1918 he produced numerous works during the final campaigns of the Western Front.
After the war, Longstaff continued his art, turning many of his sketches into paintings. Even after leaving the military, he remained in England, eventually settling in Sussex. Beginning the late 1920s, he made return trips to the battlefields of Belgium and France and painted haunting images in a spiritualist style. Among these later works is Menin Gate at Midnight (1927), arguably his most famous, which depicts the ghostly figures of soldiers marching past the monument. The painting toured Australia in 1928–29. It was viewed by record crowds.