Sussex County is a county in the southern part of the U.S. state of Delaware, on the Delmarva Peninsula. As of the 2020 census, the population was 237,378, making it the state's second most populated county only behind New Castle and ahead of Kent. The county seat is Georgetown. The first European settlement in the state of Delaware was founded by the Dutch in 1631 near the present-day town of Lewes on the Atlantic Coast.