William Stewart Agras (born 17 May 1929 London) is an American psychiatrist and psychotherapist of British origin, research psychiatrist and Emeritus (Active) Professor of Psychiatry and Behavioral Science at Stanford University.
His primary research interest is in the application of basic psychology to investigation of behavior change in medical and psychiatric problems such as treatment compliance, essential hypertension and other cardiovascular risk factors, and anxiety disorders.
Beginning in 1985 the majority of his work has been in the area of eating disorders, with studies ranging from epidemiology, basic psychological mechanisms, and treatment.
He did a residency and Fellowship in psychiatry at McGill University in Montreal, Quebec in 1956–1961.
He then served as Chair of the Department of Psychiatry at the University of Mississippi Medical Center in Jackson, Mississippi for the next four years where he developed a Department in which psychology and psychiatry were placed on an equal footing and the role of clinical psychologists became one of research and teaching.