Wentzville is an exurb of St. Louis that is located in western St. Charles County, Missouri, United States. As of the 2022 U.S. Census Bureau estimates, the city had a total population of 47,197, making it the 15th largest city in Missouri. Wentzville has been the fastest growing city in Missouri, by percentage population increase, for two consecutive decades from 2000 to 2020. As the site of Rotary Park, Wentzville is host to the St. Charles County Fair and the St. Louis Renaissance Festival.

St. Charles County is in the central eastern part of the U.S. state of Missouri. As of the 2020 census, the population was 405,262, making it Missouri's third-most populous county. Its county seat is St. Charles. The county was organized October 1, 1812, and named for Saint Charles Borromeo, an Italian cardinal.