The Undyulyung (Russian: Ундюлюнг; Yakut: Үндүлүҥ, Ündülüŋ) is a river in Sakha Republic (Yakutia), Russia. It is the 19th longest tributary of the Lena, with a length of 414 kilometres and a drainage basin area of 12,800 square kilometres. There are no settlements close to the river.
A great number of fish species are found in the waters of the Undyulyung, including lenok, taimen, whitefish, perch, grayling, pike, ide, as well as the endangered Lena sturgeon. The river is a suitable destination for rafting.[
