Wentzville is a city located in western St. Charles County, Missouri, United States. As of the 2010 census, the city had a total population of 29,070. The city's major employer is General Motors which has a full size van assembly plant located there, called Wentzville Assembly. As the site of the county fairgrounds, Wentzville hosts the annual St. Charles County Fair, St. Louis Pirate Festival and the Greater St. Louis Renaissance Faire.