The National Security Act of 1947 signed July 26, 1947 by U.S. President Harry S. Truman realigned and reorganized the United States' armed forces, foreign policy, and Intelligence Community apparatus in the aftermath of World War II.

It merged the United States Department of War and the United States Department of the Navy into the United States Department of Defense headed by the Secretary of Defense. It was also responsible for the creation of a separate United States Air Force from the existing United States Army Air Corps. Initially, each of the three branches maintained quasi-cabinet status through their individual secretaries, but the act was amended in 1949 to assure their subordination to the Secretary of Defense.

Aside from the military reorganization, the act established the National Security Council, a central place of coordination for national security policy in the Executive Branch, and the Central Intelligence Agency, the United States' first peacetime intelligence agency.

The act and its changes, along with the Truman Doctrine and the Marshall Plan, were major components of the Truman administration's Cold War strategy.