Grand Lake St. Marys State Park is a public recreation area located on 13,500-acre (5,500 ha) Grand Lake in Mercer and Auglaize counties, Ohio. Grand Lake is the largest inland lake in Ohio in terms of area, but is shallow, with an average depth of only 5–7 feet (1.5–2.1 m). The state park is open for year-round recreation, including boating, fishing, swimming and hunting. The park consists of the lake and park facilities scattered all around the shore intermingled with private property and a facility operated by Wright State University. It is west of St. Marys, and southeast of Celina, 23 miles (37 km) southwest of Lima in the northwestern part of the state.

Mercer County is located in the U.S. state of Ohio. As of the 2020 census, the population was 42,528. Its county seat is Celina. The county was created in 1820 and later organized in 1824. It is named for Hugh Mercer, an officer in the American Revolutionary War.