Klaus F. Riegel (November 6, 1925 in Berlin, Germany – July 3, 1977 in Ann Arbor, United States) was professor of psychology at the University of Michigan from 1959 to 1977.
Klaus Riegel worked as a metal worker in a shipyard until he was admitted to the University of Hamburg, Germany, where he studied under Curt Bondy (who was a student of William Stern (psychologist)) and received a degree in psychology.
In 1959, he joined the faculty at the University of Michigan Department of Psychology, where he participated in the Psycholinguistics Program, the Institute of Gerontology, and the Center for Human Growth and Development.
He published and edited numerous journal articles on psycholinguistics, the history and philosophy of the social sciences, and the development and assessment of intellectual functions of older adults.
Research from the cross-sectional study became the basis for the terminal drop hypothesis, in which a decline in cognitive performance or behavior occurs in older adults five years prior to death.