Simon Flexner ForMemRS[1] (March 25, 1863 – May 2, 1946) was a physician, scientist, administrator, and professor of experimental pathology at the University of Pennsylvania (1899–1903).

Simon was born in Louisville, Kentucky, to Moritz (Morris) Flexner, a Jewish immigrant from Neumark, Bohemia, via several years in Strasbourg, France; and Ester from Roden, Germany.

In December 1907 Flexner declared in a reading of his paper on "Tendencies in Pathology" in the University of Chicago that it would be possible in the then-future for diseased human organs substitution for healthy ones by surgery—including arteries, stomach, kidneys and heart.

His son James Thomas Flexner became a prolific writer; one of his works was an extensive biography of George Washington.

His papers are currently housed at the American Philosophical Society[10] and the Becker Medical Library at the Washington University School of Medicine.