Lewis County is a county located in the northeastern portion of the U.S. state of Missouri. As of the 2020 census, the population was 10,032. Its county seat is Monticello. The county was organized January 2, 1833, and named for Meriwether Lewis, the explorer and Governor of the Louisiana Territory.

Monticello is a rural village in, and county seat of, Lewis County, Missouri, United States, along the North Fabius River. The population was 104 at the 2020 census, and according to this census, Monticello is the county seat with the smallest population in the State of Missouri. The town is named in honor of President Thomas Jefferson's estate in Virginia. Monticello is part of the Quincy, IL–MO Micropolitan Statistical Area.