Stefan Pieńkowski (28.07.1883-20.02.1953) was a Polish experimental physicist and a member of the Polish Academy of Sciences. He was born in Młynów. From 1919 he was professor at the University of Warsaw (rector in 1925-26, 1933-36 and 1945-47) and from 1921 also director of the Institute of Experimental Physics which he organised at the University of Warsaw. In 1953 he was director of the Institute of Physics of the Polish Academy of Sciences.
His work concerned photoluminescence, X-ray structure analysis, radioactivity and the study of the Raman scattering of light.
He was president of the Polish Physical Society. He created a school of physicists.