Bertram Sidney Thomas was an English civil servant who is the first documented Westerner to cross the Rub' al Khali (Empty Quarter). He was also a scientist who practiced craniofacial anthropometry.
He was born in Pill near Bristol and educated at Trinity College, Cambridge. After working for the Civil Service in the General Post Office, he served in Belgium during World War I before being posted to the Somerset Light Infantry in Mesopotamia (now Iraq) between 1916 and 1918. He was appointed as Finance Minister and Wazir to the Sultan of Muscat and Oman (now Oman), a post he held from 1925 to 1932. In this capacity, he undertook a number of expeditions into the desert, and it was within this period that he became the first European to cross the Rub' Al Khali from 1930 and 1931.