J. Robert Oppenheimer (; April 22, 1904 – February 18, 1967) was an American theoretical physicist. A professor of physics at the University of California, Berkeley, Oppenheimer was the wartime head of the Los Alamos Laboratory and is often credited as the "father of the atomic bomb" for his role in the Manhattan Project—the World War II undertaking that developed the first nuclear weapons. Oppenheimer was among those who observed the Trinity test in New Mexico, where the first atomic bomb was successfully detonated on July 16, 1945. He later remarked that the explosion brought to mind words from the Bhagavad Gita: "Now I am become Death, the destroyer of worlds." In August 1945, the weapons were used in the atomic bombings of Hiroshima and Nagasaki.

After the war ended, Oppenheimer became chairman of the influential General Advisory Committee of the newly created United States Atomic Energy Commission. He used that position to lobby for international control of nuclear power to avert nuclear proliferation and a nuclear arms race with the Soviet Union. He opposed the development of the hydrogen bomb during a 1949–1950 governmental debate on the question and subsequently took stances on defense-related issues that provoked the ire of some U.S. government and military factions. During the Second Red Scare, those stances, together with past associations Oppenheimer had with people and organizations affiliated with the Communist Party, led to the revocation of his security clearance in a much-written-about hearing in 1954. Effectively stripped of his direct political influence, he continued to lecture, write, and work in physics. Nine years later, President John F. Kennedy awarded (and Lyndon B. Johnson presented) him with the Enrico Fermi Award as a gesture of political rehabilitation. In 2022, five decades after his death, the U.S. government formally nullified its 1954 decision and affirmed Oppenheimer's loyalty.

Oppenheimer's achievements in physics included the Born–Oppenheimer approximation for molecular wave functions, work on the theory of electrons and positrons, the Oppenheimer–Phillips process in nuclear fusion, and the first prediction of quantum tunneling. With his students he also made important contributions to the modern theory of neutron stars and black holes, as well as to quantum mechanics, quantum field theory, and the interactions of cosmic rays. As a teacher and promoter of science, he is remembered as a founding father of the American school of theoretical physics that gained world prominence in the 1930s. After World War II, he became director of the Institute for Advanced Study in Princeton, New Jersey.