Cassopolis is a village in the U.S. state of Michigan and the county seat of Cass County. The village and county are named after statesman Lewis Cass, a New Hampshire native and a prominent U.S. senator from Michigan prior to the American Civil War.Diamond Lake, one of the 100 largest inland lakes in Michigan, is located in Cassopolis.Cassopolis is part of the South Bend–Mishawaka, IN-MI, Metropolitan Statistical Area. The population was 1,774 at the 2010 census.