Monsanto Lake Beach is the larger of the two designated swimming beaches at St. Joe State Park, the 8,000-plus-acre Missouri State Parks property on the south side of Park Hills in St. Francois County. The beach sits on the north shore of Monsanto Lake, the biggest of the park's four lakes at roughly 25 acres and about 30 feet deep at its deepest point. The park occupies former lead-mining ground in the Old Lead Belt, and the surrounding hills are a mix of forest, native grassland and old mine tailings.

The park entrance is at 2800 Pimville Road in Park Hills. From the entrance, the park road leads to the Monsanto Lake day-use area, where a paved lot with four accessible spaces sits a short walk from the sand. Concrete walkways run from the parking area to the water's edge, the change house, the vault toilet and Shelter 6. A paved ramp with a gentle slope gives wheelchair users a way into the water.

The swim area is a sunny sand beach facing south across the lake. There are no lifeguards, and the park's rule is to swim at your own risk. The lake gets deep (around 30 feet in the middle), so keep children close and stay near the beach rather than striking out across open water. Cans, glass bottles, coolers, fires, alcohol and pets are all banned on the beach. Certified scuba divers may dive Monsanto Lake with a certified partner, a location marker and a check-in at the park office, so watch for dive markers. Boats on the lake are limited to paddle craft and electric motors, and a boat ramp and floating fishing dock are close to the swim area.

There is no charge to swim. Missouri does not charge entrance or day-use fees at its state parks, which are funded by the state parks, soils and water sales tax. Only the large shelter costs money, and only if you reserve it. The beach runs from Memorial Day weekend through Labor Day weekend, and on-season gate hours for the lakes and day-use areas are 7 a.m. to 8 p.m. daily. Outside that window the beach is posted closed for the season.

The Missouri Department of Natural Resources samples the water at state park beaches every week during the summer season. When a sample tops 190 E. coli colonies per 100 milliliters, staff post signs saying swimming is not recommended, so check the department's beach status page before you go. This beach has been closed for high bacteria in the past. The park also posts a lead advisory for the mine tailings area used by off-road vehicles; it advises limiting time there, washing hands and face before eating, and knocking dust off clothes and gear before leaving, especially with children age 6 and under.

Beside the sand are benches, a sand volleyball court and the reservable shelter, which seats 40 and has a large group grill. The Lakeview Trail starts nearby, paved for its first two-tenths of a mile with lake-view rest spots, and the Monsanto Lake Water Trail is marked for paddlers. Pim Lake Beach, the park's smaller second beach, is a short drive west, and the park has two campgrounds, paved bike and equestrian trails, and its large off-road vehicle riding area. Missouri Mines State Historic Site, in the old lead mill buildings, sits next to the park.