At 997 acres, Thorn Creek Nature Preserve lies at the headwaters of Thorn Creek and consists of bottomlands, ravines, and white-oak forest interspersed with marsh and meadows. The preserve began as a 5-acre purchase in 1974, and the District has partnered with the Illinois Department of Natural Resources, Village of Park Forest, and Village of University Park to expand and protect this high-quality natural area. 

In 1978, 1993, and 2001, portions of the site were dedicated as a State Nature Preserve in recognition of the diverse habitats for a number of state-listed species. The joint owners of the preserve are continuously working to enhance and restore the preserve through natural resource management activities such as invasive plant control and native plant seeding.

Located within the preserve is Thorn Creek Nature Center, which was converted from a historic 1862 church and now houses a variety of exhibits, nature displays, maps, photographic studies, and a research library. The Nature Center also offers a variety of nature-oriented programs for families and adults.

Thorn Creek's 2.5 miles of hiking trails display the loveliness of the preserve's surroundings and are a must see. The trails include a trip around Owl Lake and along a boardwalk through the marsh. Because of the sensitive natural resources, dogs are not allowed in the preserve.