Lieutenant General Leslie Richard Groves Jr. (17 August 1896 – 13 July 1970) was a United States Army Corps of Engineers officer who oversaw the construction of the Pentagon and directed the Manhattan Project, a top secret research project that developed the atomic bomb during World War II.

The son of a U.S. Army chaplain, Groves lived at various Army posts during his childhood. In 1918, he graduated fourth in his class at the U.S. Military Academy at West Point and was commissioned into the U.S. Army Corps of Engineers. In 1929, he went to Nicaragua as part of an expedition to conduct a survey for the Inter-Oceanic Nicaragua Canal. Following the 1931 earthquake, Groves took over Managua's water supply system, for which he was awarded the Nicaraguan Presidential Medal of Merit. He attended the Command and General Staff School at Fort Leavenworth, Kansas, in 1935 and 1936; and the Army War College in 1938 and 1939, after which he was posted to the War Department General Staff. Groves developed "a reputation as a doer, a driver, and a stickler for duty" and in 1940 he became special assistant for construction to the Quartermaster General, tasked with inspecting construction sites and checking on their progress. In August 1941, he was appointed to create the gigantic office complex for the War Department's 40,000 staff that would ultimately become the Pentagon.

In September 1942, Groves took charge of the Manhattan Project. He was involved in most aspects of the atomic bomb's development: he participated in the selection of sites for research and production at Oak Ridge, Tennessee; Los Alamos, New Mexico; and Hanford, Washington. He directed the enormous construction effort, made critical decisions on the various methods of isotope separation, acquired raw materials, directed the collection of military intelligence on the German nuclear energy project and helped select the cities in Japan that were chosen as targets. Groves wrapped the Manhattan Project in security but failed to prevent the Soviet Union from conducting a successful espionage program that stole some of its most important secrets.

After the war, Groves remained in charge of the Manhattan Project until responsibility for nuclear weapons production was handed over to the United States Atomic Energy Commission in 1947. He then headed the Armed Forces Special Weapons Project, which had been created to control the military aspects of nuclear weapons. He was given a dressing down by the Army Chief of Staff, General of the Army Dwight D. Eisenhower, and told that he would never be appointed Chief of Engineers. Three days later, Groves announced his intention to leave the Army. He was promoted to lieutenant general just before his retirement on 29 February 1948 in recognition of his leadership of the bomb program. By a special Act of Congress, his date of rank was backdated to 16 July 1945, the date of the Trinity nuclear test. He went on to become a vice-president at Sperry Rand.