Lensk is a town situated in the Sakha Republic of Russia, serving as the administrative center of Lensky District. It lies on the left bank of the Lena River at the southern edge of the Lena Plateau. The settlement is located approximately 840 kilometers west of Yakutsk, the capital of the republic. Within the framework of administrative divisions, it functions as a town under district jurisdiction and is incorporated within Lensky District as the Town of Lensk.

The original settlement was founded in 1663 by Russian fur traders on the site of an older Evenk settlement known as Mukhtuy. The name Mukhtuya derived from an Evenk term meaning great water. During the nineteenth and early twentieth centuries, it served as a place of political exile. Rapid growth occurred during the twentieth century due to diamond deposits in the Vilyuy River basin, leading to its renaming after the river when granted town status on July 13, 1963.

The town is characterized by its role as a major base for construction associated with the Mir Mine and the nearby town of Mirny. A significant historical event occurred in May 2001 when almost the entire town was flooded due to river ice blocking the downstream flow, destroying many buildings before the town was largely rebuilt. The area also features natural landmarks, including a karst cave with a twenty-five-meter waterfall and a karst lake located seven kilometers southwest of the town.

Cultural attractions include a history museum and a branch of the Irkutsk Polytechnic Institute. The town serves as a processing point for the regional diamond industry and hosts the Yakutalmaz scientific research organization. Visitors can explore the local heritage through these institutions, which highlight the region's geological significance and historical development from a fur trading post to an industrial hub connected to major mining operations.

As of 2025, the population is 21,007. The economy relies on the diamond industry, forestry, timber, and a large-panel housing factory. Transport links include roads connecting to Mirny and scheduled air connections from Lensk Airport to Mirny, Yakutsk, and Irkutsk. The climate is strongly continental subarctic with severely cold winters averaging minus twenty-eight point three degrees Celsius in December and brief, warm summers averaging eighteen degrees Celsius in July.