Union Star is a city in northwest DeKalb County, Missouri, United States, along the Third Fork of the Platte River. The population was 380 at th…Union Star is a city in northwest DeKalb County, Missouri, United States, along the Third Fork of the Platte River. The population was 380 at the 2020 census. It is part of the St. Joseph, MO–KS Metropolitan Statistical Area.