Sir Joseph Rotblat KCMG CBE FRS (Polish: Józef Rotblat; 4 November 1908 – 31 August 2005) was a Polish and British physicist. During World War II he worked on Tube Alloys and the Manhattan Project, but left the Los Alamos Laboratory on grounds of conscience after it became clear to him in 1944 that Germany had ceased development of an atomic bomb.

His work on nuclear fallout was a major contribution toward the ratification of the 1963 Partial Nuclear Test Ban Treaty. A signatory of the 1955 Russell–Einstein Manifesto, he was secretary-general of the Pugwash Conferences on Science and World Affairs from their founding until 1973 and shared, with the Pugwash Conferences, the 1995 Nobel Peace Prize "for efforts to diminish the part played by nuclear arms in international affairs and, in the longer run, to eliminate such arms."

Józef Rotblat was born on 4 November 1908 to a Polish-Jewish family in Warsaw, then part of the Russian-ruled Kingdom of Poland, better known as Congress Poland. He was one of seven children, two of whom died in infancy. His father, Zygmunt Rotblat, built up and ran a nationwide horse-drawn carriage business, owned land and bred horses. Józef's early years were spent in what was a prosperous household but circumstances changed at the outbreak of World War I. Borders were closed and the family's horses were requisitioned, leading to the failure of the business and poverty for their family. Despite having a religious background, by the age of ten, he doubted the existence of God, and later became an agnostic.

Rotblat's parents could not afford to send him to a gymnasium, so Rotblat received his secondary education in a cheder taught by a local rabbi. He then attended a technical school, where he studied electrical engineering, graduating with his diploma in 1923 in the newly established Republic of Poland. After graduating, Rotblat worked as an electrician in Warsaw, but had an ambition to become a physicist. He sat the entrance examinations of the Free University of Poland in January 1929, and passed the physics one with ease, but was less successful in writing a paper about the Commission of National Education, a subject about which he knew nothing. He was then interviewed by Ludwik Wertenstein [pl], the Dean of the Science Faculty. Wertenstein had studied in Paris under Marie Curie and at the Cavendish Laboratory at the University of Cambridge under Ernest Rutherford. Wertenstein offered Rotblat a place.

Rotblat earned a Master of Arts at the Free University in 1932. After, he entered the University of Warsaw, and became a Doctor of Physics in 1938. He held the position of Research Fellow in the Radiological Laboratory of the Scientific Society of Warsaw, of which Wertenstein was the director, and became assistant Director of the Atomic Physics Institute of the Free University of Poland in 1938.

During this period, Rotblat married a literature student, Tola Gryn, whom he had met at a student summer camp in 1930.

Before the outbreak of World War II, he conducted experiments that showed that in the fission process, neutrons were emitted. In early 1939, he envisaged that a large number of fissions could occur and if this happened within a sufficiently short time, then considerable amounts of energy could be released. He went on to calculate that this process could occur in less than a microsecond, and as a consequence would result in an explosion.

In 1939, through Wertenstein's connections, Rotblat was invited to study in Paris and at the University of Liverpool under James Chadwick, winner of the 1932 Nobel Prize for discovering the neutron. Chadwick was building a particle accelerator called a "cyclotron" to study fundamental nuclear reactions, and Rotblat wanted to build a similar machine in Warsaw, so he decided to join Chadwick in Liverpool. He travelled to England alone because he could not afford to support his wife there.