Nodaway County is a county located in the northwest part of the U.S. state of Missouri. As of the 2010 census, the population was 23,370. Its county seat is Maryville. The county was organized February 14, 1845, and is named for the Nodaway River. It is the largest in area of the counties added to Missouri in the 1836 Platte Purchase, and the fourth-largest county in Missouri.