Kwajalein Atoll (/ˈkwɑːdʒɨlɨn/; Marshallese: Kuwajleen /kʷɨɦˠʷatʲlʲɜɦʲɜnʲ/ or [kʷuwɒ͡æʑælɛɛ̯ɛn]), is part of the Republic of the Marshall Islands (RMI). The southernmost and largest island in the atoll is named Kwajalein Island, which English-speaking residents often call by the shortened name, Kwaj (/ˈkwɑːdʒ/).

The atoll lies in the Ralik Chain, 2,100 nautical miles (3900 km) southwest of Honolulu, Hawaii, at 8°43′N 167°44′E / 8.717°N 167.733°E / 8.717; 167.733.

Kwajalein is one of the world's largest coral atolls as measured by area of enclosed water. Comprising 97 islands and islets, it has a land area of 16.4 km² (6.33 mi²), and surrounds one of the largest lagoons in the world, with an area of 2174 km² (839 mi²). The average height above sea level for all the islands is about 1.8 metres (5 ft 11 in).

Kwajalein Island is the southernmost, and the largest, of the islands in the Kwajalein atoll. The northernmost, and second largest, island is Roi-Namur.^{[citation needed]}

The island is about 1.2 square miles (3.1 km²). It is 2.5 miles (4.0 km) long and averages about 800 yards (730 m) wide.