Simon Flexner, American physician and academic (d. 1946)
Simon Flexner, M.D. (March 25, 1863 in Louisville, Kentucky – May 2, 1946) was a physician, scientist, administrator, and professor of experimental pathology at the University of Pennsylvania (1899–1903). He served as the first director of the Rockefeller Institute for Medical Research (1901–1935) (later developed as Rockefeller University) and a trustee of the Rockefeller Foundation. He was also a friend and adviser to John D. Rockefeller Jr.

Among Flexner's most important achievements are studies into poliomyelitis and the development of serum treatment for meningitis. Among his lab assistants were Hideyo Noguchi and Cornelius Rhoads, later directors of Memorial Hospital and the Sloan-Kettering Institute, respectively.

The bacteria species Shigella flexneri was named in recognition of Flexner. In addition, Flexner was the first to describe Flexner-Wintersteiner rosettes, a characteristic finding in retinoblastoma, a type of cancer.