Hillsboro is a city and county seat of Jefferson County, Missouri, United States. The population was 3,473 at the 2020 census, up from 2,821 in 2010.

Jefferson County is located in the eastern portion of the U.S. state of Missouri. It is a part of the St. Louis Metropolitan Area. As of the 2020 census, the population was 226,739, making it the sixth-most populous county in Missouri. Its county seat is Hillsboro. The county was organized in 1818 and named in honor of former president Thomas Jefferson.