Ulithi (Wulthiy, Yulthiy, or Wugöy; pronounced roughly as YOU-li-thee) is an atoll in the Caroline Islands of the western Pacific Ocean, about east of Yap.
Ulithi consists of 40 islets totaling , surrounding a lagoon about long and up to wide—at one of the largest in the world. It is administered by the state of Yap in the Federated States of Micronesia. Ulithi's population was 773 in 2000. There are four inhabited islands on Ulithi Atoll. They are Falalop (Fl'aalop), Asor (Yasor), Mogmog (Mwagmwog), and Fedarai (Fedraey). Falalop is the most accessible with Ulithi Airport, a small resort hotel, store and one of three public high schools in Yap state. Mogmog is the seat of the high chief of Ulithi Atoll though each island has its own chief. Other important islands are Losiap (L'oosiyep), Sorlen (Sohl'oay), and Potangeras (Potoangroas).
The atoll is in the westernmost of the Caroline Islands, southwest of Guam, east of the Philippines and south of Tokyo. It is a typical volcanic atoll, with a coral reef, white sand beaches and palm trees. Ulithi's forty small islands barely rise above the sea, with the largest being only in area. However the reef runs roughly north and south, by across, enclosing a vast anchorage with an average depth of .
The Portuguese navigator Diogo da Rocha is credited as the first European to find Ulithi in 1525. The Spanish navigator Álvaro de Saavedra arrived on the ship Florida on 1 January 1528, claiming the islands for King Philip II under the name Islands of the Kings (Islas de los Reyes; Îles des Rois) after his patron and the Three Wise Men honored in the approaching Catholic feast of Epiphany. It was later charted by other Spaniards as the Chickpea Islands (Islas de los Garbanzos). It was also visited by the Spanish expedition of Ruy López de Villalobos on 26 January 1543.
It remained isolated until visited and explored in detail by Captain Don Bernardo de Egoy in 1712, and later visited by Spanish Jesuit missionaries led by Juan Antonio Cantova together with a group of 12 Spanish soldiers in 1731.