The United States National Nuclear Security Administration (NNSA) is part of the United States Department of Energy. It works to improve national security through the military application of nuclear energy. The NNSA also maintains and improves the safety, reliability, and performance of the United States nuclear weapons stockpile, including the ability to design, produce, and test, in order to meet national security requirements.

The National Nuclear Security Administrations was created by Congressional action in 1999, in the wake of the Wen Ho Lee spy scandal and other allegations that lax administration by the Department of Energy had resulted in the loss of U.S. nuclear secrets to China. Originally proposed to be an independent agency, NNSA gained the reluctant support of the Clinton Administration only after it was instead chartered as a sub-agency within the Department of Energy, to be headed by an Administrator reporting to the Secretary of Energy. The first NNSA Administrator appointed was Air Force General (and CIA Deputy Director) John A. Gordon.