Westfield is a city in Plymouth County, Iowa, United States. The population was 123 at the time of the 2020 census. It is the westernmost point i…Westfield is a city in Plymouth County, Iowa, United States. The population was 123 at the time of the 2020 census. It is the westernmost point in Iowa, this being due to a bend in the Big Sioux River. The nearby Broken Kettle wildlife refuge is one of the few places in Iowa where prairie rattlesnakes are commonly found.