Klaus Emil Julius Fuchs (29 December 1911 – 28 January 1988) was a German theoretical physicist, atomic spy, and communist who supplied information from the American, British, and Canadian Manhattan Project to the Soviet Union during and shortly after World War II. While at the Los Alamos Laboratory, Fuchs was responsible for many significant theoretical calculations relating to the first nuclear weapons and, later, early models of the hydrogen bomb. After his conviction in 1950, he served nine years in prison in the United Kingdom, then migrated to East Germany where he resumed his career as a physicist and scientific leader.

The son of a Lutheran pastor, Fuchs attended the University of Leipzig, where his father was a professor of theology, and became involved in student politics, joining the student branch of the Social Democratic Party of Germany (SPD), and the Reichsbanner Schwarz-Rot-Gold, an SPD-allied paramilitary organisation. He was expelled from the SPD in 1932, and joined the Communist Party of Germany (KPD). He went into hiding after the 1933 Reichstag fire and the subsequent persecution of communists in Nazi Germany, and fled to the United Kingdom, where he received his PhD from the University of Bristol under the supervision of Nevill Francis Mott, and his DSc from the University of Edinburgh, where he worked as an assistant to Max Born.

After the Second World War broke out in Europe, he was interned in the Isle of Man, and later in Canada. After he returned to Britain in 1941, he became an assistant to Rudolf Peierls, working on "Tube Alloys"—the British atomic bomb project. He began passing information on the project to the Soviet Union through Ursula Kuczynski, codenamed "Sonya", a German communist and a major in Soviet military intelligence who had worked with Richard Sorge's spy ring in the Far East. In 1943, Fuchs and Peierls went to Columbia University, in New York City, to work on the Manhattan Project. In August 1944, Fuchs joined the Theoretical Physics Division at the Los Alamos Laboratory, working under Hans Bethe. His chief area of expertise was the problem of implosion, necessary for the development of the plutonium bomb. After the war, he returned to the UK and worked at the Atomic Energy Research Establishment at Harwell as head of the Theoretical Physics Division.

In January 1950, Fuchs confessed that he had passed information to the Soviets over a seven-year period beginning in 1942. A British court sentenced him to fourteen years' imprisonment and he was subsequently stripped of his British citizenship. He was released in 1959, after serving nine years, and migrated to the German Democratic Republic (East Germany), where he was elected to the Academy of Sciences and became a member of the Socialist Unity Party of Germany (SED) central committee. He was later appointed deputy director of the Central Institute for Nuclear Physics in Dresden, where he served until his retirement in 1979.

Post Cold War declassified information states that the Russians freely acknowledged that Fuchs gave them the fission bomb.

Klaus Emil Julius Fuchs was born in Rüsselsheim, Grand Duchy of Hesse, on 29 December 1911, the third of four children of a Lutheran pastor, Emil Fuchs, and his wife Else Wagner. His father served in the army during World War I but later became a pacifist and a socialist, joining the Social Democratic Party of Germany (SPD) in 1912. He eventually became a Quaker. Fuchs had an older brother Gerhard, an older sister Elisabeth, and a younger sister, Kristel. The family moved to Eisenach, where Fuchs attended the Martin-Luther Gymnasium, and took his Abitur. At school, Fuchs and his siblings were taunted over his father's unpopular socialist political views, which they came to share. They became known as the "red foxes", Fuchs being the German word for fox.

Fuchs was left-handed, but was forced to write with his right hand.

Fuchs entered the University of Leipzig in 1930, where his father was a professor of theology. He became involved in student politics, joining the student branch of the SPD, a party that his father had joined in 1921, and the Reichsbanner Schwarz-Rot-Gold, the party's paramilitary organisation. His father took up a new position as professor of religion at the Pedagogical Academy in Kiel, and in the autumn Fuchs transferred to the University of Kiel, which his brother Gerhard and sister Elisabeth also attended. Fuchs continued his studies in mathematics and physics at the university. In October 1931, his mother committed suicide by drinking hydrochloric acid. The family later discovered that his maternal grandmother had also taken her own life.