Nizhneudinsk is a town located in Russia and serves as the administrative center of the Nizhneudinsky District within Irkutsk Oblast. It is situated on the Uda River, approximately 506 kilometers (314 miles) northwest of Irkutsk, which is the administrative center of the oblast.
According to census data, the population of the town has fluctuated over recent decades. The 2010 census recorded a population of 36,999, following a figure of 39,700 in 1970 and 43,759 during the 1989 Soviet census.