Platte County is a county located in the northwestern portion of the U.S. state of Missouri. As of the 2010 census, the population was 89,322. Its county seat is Platte City. The county was organized December 31, 1838 from the Platte Purchase, named for the Platte River. (Platte is derived from the French word for a low, shallow, or intermittent stream.)Platte County is part of the Kansas City, MO-KS Metropolitan Statistical Area and contains some of the city's northwestern suburbs.