Kwajalein Atoll (; Marshallese: Kuwajleen [kʷuwɑzʲ(ɛ)lʲɛːnʲ]) is part of the Republic of the Marshall Islands (RMI). The southernmost and largest island in the atoll is named Kwajalein Island, which its majority English-speaking residents (about 1,000 mostly U.S. civilian personnel) often use the shortened name, Kwaj . The total land area of the atoll amounts to just over 6 square miles (16 km2). It lies in the Ralik Chain, 2,100 nautical miles (3,900 km; 2,400 mi) southwest of Honolulu, Hawaii.

The US Navy has hosted a naval base on Kwajalein Island since World War II. It was the final resting place of the German cruiser Prinz Eugen after it survived the Operation Crossroads nuclear test in 1946. In the late 1950s, the US Army took over the base as part of their Nike Zeus anti-ballistic missile efforts, and since then the atoll has been widely used for missile tests of all sorts. Today it is part of the Ronald Reagan Ballistic Missile Defense Test Site, with various radars, tracking cameras, missile launchers and many support systems spread across many islands. One of the five ground stations used in controlling the operation of the Global Positioning System (GPS) navigation system is located on Kwajalein.