Trinity was the first detonation of a nuclear weapon, conducted by the United States Army at 5:29 a.m. Mountain War Time (11:29:21 GMT) on July 16, 1945, as part of the Manhattan Project. The test was of an implosion-design plutonium bomb, or "gadget" – the same design as the Fat Man bomb later detonated over Nagasaki, Japan, on August 9, 1945. Concerns about whether the complex Fat Man design would work led to a decision to conduct the first nuclear test. The code name "Trinity" was assigned by J. Robert Oppenheimer, the director of the Los Alamos Laboratory; the name was possibly inspired by the poetry of John Donne.

Planned and directed by Kenneth Bainbridge, the test was conducted in the Jornada del Muerto desert about 35 miles (56 km) southeast of Socorro, New Mexico, on what was the Alamogordo Bombing and Gunnery Range, but was renamed the White Sands Proving Ground just before the test. The only structures originally in the immediate vicinity were the McDonald Ranch House and its ancillary buildings, which scientists used as a laboratory for testing bomb components.

Fears of a fizzle prompted construction of "Jumbo", a steel containment vessel that could contain the plutonium, allowing it to be recovered, but Jumbo was not used in the test. On May 7, 1945, a rehearsal was conducted, during which 108 short tons (98 t) of high explosive spiked with radioactive isotopes was detonated.

425 people were present on the weekend of the Trinity test. In addition to Bainbridge and Oppenheimer, observers included Vannevar Bush, James Chadwick, James B. Conant, Thomas Farrell, Enrico Fermi, Hans Bethe, Richard Feynman, Isidor Isaac Rabi, Leslie Groves, Frank Oppenheimer, Geoffrey Taylor, Richard Tolman, Edward Teller, and John von Neumann. The Trinity bomb released the explosive energy of 25 kilotons of TNT (100 TJ) ± 2 kilotons of TNT (8.4 TJ), and a large cloud of fallout. Thousands of people lived closer to the test than would have been allowed under guidelines adopted for subsequent tests, but no one living near the test was evacuated before or afterward.

The test site was declared a National Historic Landmark district in 1965 and listed on the National Register of Historic Places the following year.

The creation of nuclear weapons arose from the scientific and political developments of the 1930s. The decade saw many new discoveries about the nature of atoms, including the existence of nuclear fission. The concurrent rise of fascist governments in Europe led to a fear of a German nuclear weapon project, especially among scientists who were refugees from Nazi Germany and other fascist countries. When their calculations showed that nuclear weapons were theoretically feasible, the British and United States governments supported an all-out effort to build them.

These efforts were transferred to the authority of the U.S. Army in June 1942 and became the Manhattan Project. Brigadier General Leslie R. Groves, Jr. was appointed its director in September. The weapons development portion of this project was located at the Los Alamos Laboratory in northern New Mexico, under the directorship of physicist J. Robert Oppenheimer. The University of Chicago, Columbia University, and the Radiation Laboratory at the University of California, Berkeley, conducted other development work.

Manhattan Project scientists had identified two fissile isotopes for potential use in bombs: uranium-235 and plutonium-239. Uranium-235 became the basis of the Little Boy bomb design, first used (without prior testing) in the bombing of Hiroshima; the design used in the Trinity test, and eventually used in the bombing of Nagasaki (Fat Man), was based on plutonium. The original design considered for a weapon based on plutonium-239 was Thin Man, in which (as in the Little Boy uranium bomb) two subcritical masses of fissile material would be brought rapidly together to form a single critical mass.