Wentzville is a city located in western St. Charles County, Missouri, United States. As of 2009, the city had a total population of 28,768. Wentzville is currently Missouri's fastest growing city. The city's major employer is General Motors which has a full size van assembly plant located there. Site of the county fairgrounds, Wentzville annually hosts the St. Charles County Fair, St. Louis Pirate Festival and the Greater St. Louis Renaissance Faire.