William Rees-Thomas CB FRCP FRSM (15 June 1887 – 13 April 1978) was a Welsh psychiatrist. He was Medical Senior Commissioner for the Board of Control for Lunacy and Mental Deficiency. Born in , Breconshire, he was educated at and Cardiff University. He became a Member of the Royal College of Physicians in 1913 and a Fellow of the Royal College of Physicians in 1933. He was awarded the , 1906; , 1907; , 1909; (RCP), 1912; and gold medal. During the First World war he served in the Royal Army Medical Corps He was made a Companion of the Order of the Bath (CB) in 1950