West Union is a city in Fayette County, Iowa, United States. The population was 2,490 at the time of the 2020 census. It is the county …West Union is a city in Fayette County, Iowa, United States. The population was 2,490 at the time of the 2020 census. It is the county seat of Fayette County. Originally called Knob Prairie, the community was founded by William Wells, naming it for his hometown, also called West Union, in Ohio. The vacuum cleaner was invented here.