The National Optical Astronomy Observatory (NOAO) is the United States national observatory for ground-based nighttime ultraviolet-optical-infrared (OUVIR) astronomy. The National Science Foundation (NSF) funds NOAO to provide forefront astronomical research facilities for US astronomers. However, professional astronomers from any country in the world may apply to use the telescopes operated by NOAO under the NSF's "open skies" policy. Astronomers submit proposals for peer review to gain access to the telescopes which are scheduled every night of the year for observations (with the exception of Christmas and New Year's Eve). The combination of truly open access and the merit based science proposal process makes NOAO unique in the world.

NOAO is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF. Its headquarters in Tucson, Arizona are co-located with the headquarters of the National Solar Observatory. The budget for NOAO during the 2017 fiscal year was nearly $23 million.

NOAO was founded in 1984 to join the operations of the Kitt Peak National Observatory in the United States with the Cerro Tololo Inter-American Observatory in Chile. On October 1, 2019, NOAO merged its operations with the Gemini Observatory and the Vera C. Rubin Observatory to form the National Optical-Infrared Astronomy Research Laboratory (NOIRLab)..