Lake Udsu (, also , , or ) is a lake in Estonia. It is located in the village of Jeti in Tõrva Parish, Valga County. Lake Udsu is one of the deepest lakes in Estonia. Lake Udsu belongs to the Koorküla Nature Reserve.
The lake has an area of The lake has a maximum depth of . It is long, and its shoreline measures . Lake Udsu is an oligotrophic lake.