Otto Hahn (March 8, 1879 – July 28, 1968) was a German physicist. He received the 1944 Nobel Prize in Chemistry.

Hahn was born in Frankfurt am Main and studied chemistry in Marburg and Munich. After receiving his PhD in 1901 he worked initially at Marburg university then, from 1904, at London, from 1905 at Montreal and from 1906 in Berlin.

Together with Lise Meitner and Otto von Baeyer , he developed a technique to measure the beta decay spectra of radioactive isotopes, which achievement was recognised by his securing the post of professor at the newly founded Kaiser-Wilhelm-Institute for Chemistry in Berlin in 1912.

In 1918, he, together with Meitner, discovered protactinium. When Meitner fled Nazi Germany in 1938, he continued work with Fritz Straßmann on elucidating the outcome of the bombardment of uranium with thermal neutrons. He communicated his results to Meitner who, in collaboration with her nephew Otto Robert Frisch, correctly interpreted them as evidence of nuclear fission (a phrase coined by Frisch).

Once the idea of fission had been accepted, Hahn continued his experiments and demonstrated the huge amounts of energy that neutron-induced fission could produce, either for energy production or warfare.

During World War II Hahn was a participant in the German program to develop a fission weapon under the leadership of Werner Heisenberg. Hahn was awarded the 1944 Nobel Prize in Chemistry, but at the awards ceremony the chairman of the Nobel Committee for Chemistry announced, "Professor Hahn has informed us that he is regrettably unable to attend this ceremony." He was being held prisoner by the British who were seeking information from him about the failed German effort to develop an atomic bomb. In the post-war era Hahn became a popular fighter against the use of nuclear weapons.

Proposals were made at different times that each of elements 105 and 108 should be named Hahnium in Hahn's honour, but neither proposal found approval (see Element naming controversy).

The world's first nuclear-powered merchant ship, Otto Hahn, was named in his honor. Along with Fritz Straßmann and Lise Meitner, Hahn is the discoverer of Nuclear fission.