Round Lake is a village in Lake County, Illinois, United States. As of the 2010 census, it had a population of 18,289. According to the 2010 census, Round Lake has a total area of 5.635 square miles (14.59 km2), of which 5.47 square miles (14.17 km2) (or 97.07%) is land and 0.165 square miles (0.43 km2) (or 2.93%) is water.
While the retreating Wisconsin glacier left an attractive environment for farmers who entered western Lake County after the Black Hawk War of 1832, the numerous lakes and wet prairies there prevented easy movement to agricultural markets. Farmers traded at stagecoach trail communities such as Hainesville, often exchanging dairy products and eggs for what they could not craft on the farm. In the 1890s, when officials of the Chicago, Milwaukee & St. Paul Railroad extended a branch line from their Milwaukee–Chicago main line at Libertyville Junction (later Roundout) to Janesville, Wisconsin, western Lake County farmers gained easy access to Chicago.