Ulleungdo (also spelled Ulreungdo; Korean pronunciation: [ulːɯŋdo]) is a South Korean island in the Sea of Japan (East Sea). Formerly known as Dagelet to the Europeans, Ulleungdo is about 120 km (75 mi) east of the Korean Peninsula. Volcanic in origin, the rocky steep-sided island is the top of a large stratovolcano which rises from the seafloor, reaching a maximum elevation of 984 metres (3,228 ft) at Seonginbong Peak. The island consists primarily of trachyandesite rock. A major explosive eruption about 9,350 years ago reached a Volcanic Explosivity Index of 6 and deposited tephra as far as central Honshū over 800 km (500 mi) away, while producing pyroclastic flows on the island and decapitating its top to form a caldera.

Ulleungdo Island, measuring 11.3 kilometres (7.0 mi) in length and 12.4 kilometres (7.7 mi) in width.

The island of Ulleungdo has an area of 73.15 km² (28.24 sq mi) with about 10,000 inhabitants. It makes up the main part of Ulleung County, North Gyeongsang Province, South Korea. The main city of Ulleungdo is the port of Dodong, which serves as the main ferry port between Ulleungdo and the Korean mainland. Ulleungdo is a popular tourist site. The other main economic activity is fishery, including its well-known harvest of cuttlefish, which can be seen drying in the sun in many places on Ulleungdo.