The Sielbeck Forest Natural Area is a state park, forest, and natural area in Metropolis, Illinois. It features a well preserved potion of the Big Black Slough, which was a wetland-rich floodplain.

Established in 1998, the Sielbeck Forest Natural Area was saved from being turned into agricultural land like its neighboring areas all thanks to the Sielbeck family. They owned the tract of land for around a century and kept it as a wilderness area, much like how it was documented by public land surveyors in 1807. The site is home to a bottomland hardwood forest and a forested swamp filled with grey timber.

The Sielbeck Forest Natural Area is now a satellite of the Mermet Lake State Fish and Wildlife Area. Visitors can enjoy hiking, fishing, and hunting at the site.