DeWitt is a city in Clinton County, Iowa, United States. The population was 5,514 at the time of the 2020 census, which represented a 9.2% incr…DeWitt is a city in Clinton County, Iowa, United States. The population was 5,514 at the time of the 2020 census, which represented a 9.2% increase over that of 2000, making it the fastest growing city in Clinton County.