Ulithi (Yapese: Wulthiy, Yulthiy, or Wugöy) is an atoll in the Caroline Islands of the western Pacific Ocean, about 191 km (103 nautical mi) east of Yap. It consists of 40 islets totalling 4.5 km2 (1.7 sq mi), surrounding a lagoon about 36 km (22 mi) long and up to 24 km (15 mi) wide—at 548 km2 (212 sq mi) 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 (Ulithian: Fl'aalop), Asor (Yasor), Mogmog (Mwagmwog), and Fedarai (Fedraey). Falalop is the most accessible with an air strip, a small resort hotel, gas dealership, 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 (Ulithian: L'oosiyep), Sorlen (Sohl'oay), and Potangeras (Potoangroas).
Ulithi was a major staging area for the U.S. Navy in the final year of the Second World War. Several sunken warships rest at the bottom of the Ulithi lagoon, including the USS Mississinewa (AO-59), a fleet oiler which sank fully loaded. The sunken tanker was found to be seeping oil into the lagoon. The United States Navy responded, locating the tanker, tapping her storage tanks, and pumped off her oil. The cleanup operation was completed in February 2003.
The atoll offers good fishing and diving, though recent typhoons have eroded some of the reefs.
Census records can be misleading because population can fluctuate during the year because it is common for Ulithians to leave for work or school abroad and to return. This is particularly true during festive times like the Outer Island High School graduation ceremony, when the population can increase considerably. Additionally, during events like weddings and funerals, Yasor's population may double.
Electricity is now available on some islands, and the advent of video players and cell phones have brought a touch of the outside world to this very isolated atoll. Occasional diving and adventure tours visit Ulithi from Yap. Western clothing is prohibited on Mogmog island for everyone including tourists.