Stefanie Horovitz (1887–1942) (Stefania Horovitz or Stephanie Horowitz) was a Polish-Jewish chemist known for experimental work proving the existence of isotopes. Between approximately 1914-1918, she worked with Otto Hönigschmid at the Radium Institute of Vienna using analytical methods to demonstrate the first and second credible cases of isotopes in lead and thorium. Later she co-founded a home for children and young adults in need of psychological therapy. She was killed by Nazis at Treblinka extermination camp in 1942.