Mercer County is a county located in the northwestern portion of the U.S. state of Missouri. As of the 2020 census, the population was 3,538, making it the second-least populous county in Missouri. Its county seat is Princeton. The county was organized February 14, 1845, and named for General John F. Mercer of the Revolutionary War.

Princeton is the county seat and largest city of Mercer County, Missouri, United States. The population was 1,007 at the 2020 census, down from the 2010 census, which counted 1,166 people. Princeton was the birthplace of the famous frontierswoman Calamity Jane.