Gustav Theodor Fechner (April 19, 1801 – November 18, 1887), is a German experimental psychologist.

An early pioneer in experimental psychology and founder of psychophysics, he inspires many 20th century scientists and philosophers.

He is also credited with demonstrating the nonlinear relationship between psychological sensation and the physical intensity of a stimulus via the formula: "S = K Log I", which becomes known as the Weber–Fechner law.