Nodaway County is a county located in the northwest part of Missouri. As of the 2020 census, the population was 21,241. Its county seat is Maryville. The county was organized February 14, 1845, and is named for the Nodaway River. It is the largest county by area added to Missouri in the 1836 Platte Purchase and the fifth-largest county by area in Missouri. Nodaway County comprises the Maryville, MO Micropolitan Statistical Area.