Lake Bomoseen is a freshwater lake in the western part of the U.S. state of Vermont in the towns of Castleton and Hubbardton in Rutland County. It is the largest lake that lies entirely within the state's boundaries, with a surface area of approximately 2,400 acres (9.6 km²). The lake was formed by glaciation and has an average and maximum depth of 27 feet (8.2 m) and 65 feet (19.8 m), respectively.