St. Louis County is a county located in eastern Missouri, bounded by the City of St. Louis and the Mississippi River to the east, the Missouri River to the north, and the Meramec River to the south. It serves as a distinct administrative entity within the state, separate from the independent City of St. Louis. The county is included in the St. Louis, MO–IL metropolitan statistical area, functioning as a key component of the broader regional governance structure.

The area was settled by ethnic French colonists who moved west after Great Britain took over former French territory east of the Mississippi River. They founded the city of St. Louis in the late 1700s, with the US acquiring the territory in 1803 via the Louisiana Purchase. In 1877, residents of the City of St. Louis voted to separate from the county, becoming an independent city. This separation allowed the county to continue expanding as suburban development grew.

The county seat is Clayton, while Florissant is the largest city. The region features a mix of historic settlements and modern developments, including areas like Kirkwood, which was established as a planned suburb in 1853. The area retains significant historical architecture, such as the Old Courthouse, which includes a finished rotunda and dome completed by 1843. These structures reflect the county's evolution from early colonial settlements to a modern governmental hub.

St. Louis is known for its vibrant culture and affordable attractions, offering numerous free sites for tourists and residents. The city played a large part during the steamboat era due to its position at the confluence of major rivers. It also hosted the 1904 World's Fair and Summer Olympic Games, leaving a legacy of parks and buildings. The area is celebrated for its aesthetic beauty, green spaces, and iconic landmarks like the Gateway Arch.

As of the 2020 census, the population was 1,004,125, making it the most populous county in Missouri. The economy has transformed from manufacturing to a center for research in medicine and biotechnology, hosting firms like Bayer and Anheuser-Busch. The county benefits from its strategic location near major waterways and transportation routes, supporting its role as a significant economic and cultural center in the Midwest.