The Nevada National Security Site (N2S2 or NNSS), known as the Nevada Test Site (NTS) until 2010, is a United States Department of Energy (DOE) reservation located in southeastern Nye County, Nevada, about 65 miles (105 km) northwest of the city of Las Vegas. Formerly known as the Nevada Proving Grounds, the site was established in 1951 for the testing of nuclear devices. It covers approximately 1,360 square miles (3,500 km2) of desert and mountainous terrain. Nuclear weapons testing at the site began with a 1-kiloton-of-TNT (4.2 TJ) bomb dropped on Frenchman Flat on January 27, 1951. Over the subsequent four decades, over 1,000 nuclear explosions were detonated at the site. Many of the iconic images of the nuclear era come from the site.

During the 1950s, the mushroom clouds from the 100 atmospheric tests could be seen from almost 100 mi (160 km) away. The city of Las Vegas experienced noticeable seismic effects, and the mushroom clouds, which could be seen from the downtown hotels, became tourist attractions. Westerly winds routinely carried the fallout of from above-ground nuclear testing directly through St. George, Utah and southern Utah. Increases in cancers, such as leukemia, lymphoma, thyroid cancer, breast cancer, melanoma, bone cancer, brain tumors, and gastrointestinal tract cancers, were reported from the mid-1950s onward. A further 921 nuclear tests were carried out underground.

From 1986 through 1994, two years after the United States put a hold on full-scale nuclear weapons testing, 536 anti-nuclear protests were held at the site, involving 37,488 participants and 15,740 arrests, according to government records.The site contains 28 areas, 1,100 buildings, 400 miles (640 km) of paved roads, 300 miles of unpaved roads, 10 heliports, and two airstrips.

Currently, Mission Support and Test Services (MSTS) is the civilian contractor for the site's management and oversees overall operations. MSTS manages and operates the site for the National Nuclear Security Administration (NNSA). Security is provided by SOC LLC.