Otto Robert Frisch (1 October 1904–22 September 1979), Austrian-British physicist. With his collaborator Rudolf Peierls he designed the first theoretical mechanism for the detonation of an atomic bomb in 1930.

Frisch was Jewish, born in Vienna in 1904 the son of a painter and a concert pianist. He himself was talented at both but also had inherited his aunt Lise Meitner's love of physics and commenced a period of study at the University of Vienna, graduating in 1926 with some work on the effect of the newly discovered electron on salts. After some years working in relatively obscure laboratories in Germany, Frisch obtained a position in Hamburg under the Nobel Prize winning scientist Otto Stern. Here he produced novel work on the diffraction of atoms (using crystal surfaces) and also proved that the magnetic moment of the proton was much larger than had been previously supposed.

The accession of Adolf Hitler to the chancellorship of Germany in 1933 made Frisch make the decision to move to London where he joined the staff at Birkbeck College and worked with the physicist Patrick Maynard Stuart Blackett on cloud chamber technology and artificial radioactivity. He followed this with a five year stint in Copenhagen with Niels Bohr where he increasingly specialised in nuclear physics particularly neutron physics.

In 1938 he visited his aunt Lise Meitner in Stockholm. While there she received the news that Otto Hahn and Fritz Strassmann in Berlin had discovered that the collision of a neutron with a uranium nucleus produced the element barium as one of its byproducts. Hahn could not explain the result. Frisch and Meitner hypothesized that the uranium nucleus had split in two, explained the process (in terms of excessive electrical charge), estimated the energy released, coined the term fission to describe it, and theorized the potential for a chain reaction. Political restraints of the Nazi era forced the team to publish separately. Hahn's paper described the experiment and asserted that the atom had split. Meitner's and Frisch's paper explained the physics behind the phenomenon. Frisch went back to Copenhagen where he was quickly able to isolate the fragments produced by fission reactions.

In the Summer of 1939 Frisch left Denmark for what he anticipated would be a short trip to Birmingham. But the outbreak of World War II precluded his return. With war on his mind and working with the physicist Rudolf Peierls the two produced the Frisch-Peierls memorandum which was the first document to set out a process by which an atomic explosion could be generated; using separated Uranium-235 which would require a fairly small critical mass and could be made to achieve criticality using conventional explosives and create an immensely powerful detonation. The memorandum went on to predict the effects of such an explosion - from the initial blast to the resulting fallout.

This memorandum was the basis of British work on building an atomic device (the Tube Alloys project) and also that of the Manhattan Project on which Frisch worked as part of the British delegation. He went to America in 1943 having been hurriedly made a British citizen. In 1946 he returned to England to take up the post of head of the nuclear physics division of the Atomic Energy Research Establishment at Harwell, though he also spent much of the next thirty years teaching at Cambridge where he was Jacksonian Professor of Natural Philosophy and a fellow of Trinity College.

He retired from the chair in 1972 to concentrate on his books and business interests. He died in 1979.