The Battle of Kwajalein was a battle of the Pacific campaign of World War II, fought from 31 January-3 February 1944, on Kwajalein Atoll in the Marshall Islands. Employing the hard-learned lessons of the battle of Tarawa, the United States launched a successful twin assault on the main islands of Kwajalein in the south and Roi-Namur in the north. The Japanese defenders put up a stiff resistance though outnumbered and under-prepared. The determined defense of Roi-Namur left only 51 survivors of an original garrison of 3,500.

For the U.S., the battle represented both the next step in its island-hopping march to Japan and a significant moral victory because it was the first time the U.S. penetrated the "outer ring" of the Japanese Pacific sphere. For the Japanese, the battle represented the failure of the beach-line defense. Japanese defenses became prepared in depth, and the battles of Peleliu, Guam, and the Marianas proved far more costly to the U.S.

Kwajalein Atoll is in the heart of the Marshall Islands. It lies in the Ralik Chain, 2,100 nmi (2,400 mi; 3,900 km) southwest of Honolulu, Hawaii at 8°43′N 167°44′E / 8.717°N 167.733°E / 8.717; 167.733. Kwajalein is the world's largest coral atoll as measured by area of enclosed water. Comprising 97 islets, it has a land area of 1,560 acres (6.33 km²) and surrounds one of the largest lagoons in the world, measuring 324 mi² (839 km²) in size.