American sociologist and social psychologist whose fields of study included cultural change and personality development and who made important contributions to methodology.

Thomas taught sociology at the University of Chicago (1895–1918), the New School for Social Research, New York City (1923–28), and Harvard University (1936–37). His Sex and Society (1907) is considered the first fully secular work on the subject by an American sociologist. Source-Book for Social Origins (1909) and Primitive Behaviour (1937) reflect his interest in ethnography. The Unadjusted Girl (1923) is a psychological study of personality. His major work, The Polish Peasant in Europe and America, 5 vol. (1918–20), written in collaboration with Florian Znaniecki, applies the comparative method to the study of nationalities and analyzes social problems by means of personal history.