Tom George Longstaff (15 January 1875 – 27 June 1964) was an English doctor, explorer and mountaineer, most famous for being the first person to climb a summit of over 7,000 metres in elevation, Trisul, in the India/Pakistann Himalaya in 1907. He also made important explorations and climbs in Tibet, Nepal, the Karakoram, Spitsbergen, Greenland, and Baffin Island. He was president of the (British) Alpine Club from 1947 to 1949 and a founding member of The Alpine Ski Club in 1908.
Longstaff was the eldest son of Lt-Col. Llewellyn W. Longstaff OBE of Wimbledon, the first and most generous supporter of Captain Scott's National Antarctic Expedition. He was educated at Eton College, Christ Church, Oxford, and St Thomas' Hospital, London.
Longstaff was commissioned into the 1/7th Battalion of the Hampshire Regiment in 1914 and served on the General Staff at Army Headquarters, Simla, 1915-1916. He was Assistant Commandant of the Gilgit Corps of Scouts, Frontier Militia, and Special Assistant at Fort Gupis to the Political Agent in Gilgit, from 1916, and was promoted Captain in 1917, retiring from the service in 1918.
During the Second World War, he served with the 7th and 13th Battalion of the KRRC from 1939 to 1941.