Johannes (Hans) Wilhelm Geiger (September 30, 1882 - September 24, 1945) was a German physicist. Together with Walther Müller he developed the Geiger counter.

In 1902 Geiger began to study physics and mathematics in Erlangen and later attained a doctorate to 1906. In 1907 he began work with Ernest Rutherford at the University of Manchester. In 1912 he became leader of the Physical-Technical Reichsanstalt in Berlin, 1925 professor in Kiel, 1929 in Tübingen, and from 1936 in Berlin. While in Berlin he developed, together with the graduate student Walter Müller, the Geiger counter.

He discovered with John Mitchell Nuttall the Geiger-Nuttal law and performed experiments that lead to Rutherford's atomic model. He was also a member of the Uranverein (Uranium Club) in Nazi Germany, the group of German physicists who, during World War II, worked on but failed to create the German atomic bomb. (How much this failure resulted from lack of scientific progress and how much from foot-dragging due to ethical concerns remains a lively debate even today.)

His unwavering loyalty to the Nazi Party led him to betray his Jewish colleagues.