Leonor Michaelis (January 16, 1875 – October 8, 1947) was a German biochemist and physician famous for his work with Maud Menten in enzyme kinetics and Michaelis-Menten kinetics.

Born in Berlin (Germany), he studied medicine in Freiburg, where he graduated in 1897. He then moved to Berlin, where he received his doctorate the same year. Michaelis worked as assistant to Paul Ehrlich (1898–1899), Moritz Litten (1899–1902) and Ernst Victor von Leyden (1902–1906). In 1906 he started as director of the bacteriology lab in Berlins Charité clinic, becoming Professor extraordinary at Berlin University in 1908. In 1922 he moved to the Medical School of the University of Nagoya (Japan) as Professor of biochemistry, 1926 to Johns Hopkins University in Baltimore, Maryland as resident lecturer in medical research and 1929 to the Rockefeller Institute of Medical Research in New York City, where he retired 1941.

Besides his role in the formulation of the Henri-Michaelis-Menten law (1913) he discovered Janus Green as a supravital stain for mitochondria and the Michaelis-Gutman body in urinary tract infections (1902) and found that thioglycolic acid could dissolve keratin, making him the father of the permanent wave.

He died in New York City.