coral atoll, Federated States of Micronesia, in the western Pacific Ocean. It comprises roughly 40 islets.

Ulithi was probably sighted by the Portuguese in 1526, but it remained undisturbed by Europeans until 1731, when it was visited by Spanish Jesuit missionaries led by Juan Antonio Cantova. The site of a Japanese seaplane base during World War II, Ulithi was captured in 1944 by Allied forces; it served as a large U.S. naval base for the duration of the war and as a military radio outpost for several years thereafter. The atoll's inhabitants are probably of mixed Polynesian (Polynesian culture) and Micronesian (Micronesian culture) heritage and speak Ulithian, an Austronesian language (Austronesian languages). Total land area 1.75 square miles (4.5 square km). Pop. (2000) 773.