Shelby County is a county located in the northeastern portion of the U.S. state of Missouri. As of the 2020 census, the population was 6,103. Its county seat is Shelbyville. The county was established on January 2, 1835, and named for Governor Isaac Shelby of Kentucky.

Isaac Shelby was the first and fifth Governor of Kentucky and served in the state legislatures of Virginia and North Carolina. He was also a soldier in Lord Dunmore's War, the American Revolutionary War, and the War of 1812. While governor, he led the Kentucky militia in the Battle of the Thames, an action that was rewarded with a Congressional Gold Medal. Counties in nine states, and several cities and military bases, have been named in his honor. His fondness for John Dickinson's "The Liberty Song" is believed to be the reason Kentucky adopted the state motto "United we stand, divided we fall".