The Smith-Duncan House and Eastman Barn are two historic buildings located on the Duncan Farmstead at Pere Marquette State Park in Jersey County, Illinois. The Smith-Duncan House is a two-story limestone house built circa 1861. The house has a double-pile plan, in which each story is two rooms deep, with a central hall. The Eastman Barn has three interior sections and is built on a raised limestone foundation; this arrangement allowed for threshing and storage to be done in the barn and provided a basement space for livestock. A gabled cupola on the barn's roof allows for air to vent from the structure. Both buildings, as well as two contributing retaining walls on the property, are well-preserved examples of local stonework; limestone was a common building material in the Grafton area during the mid-19th century.
The buildings were added to the National Register of Historic Places on November 22, 1999.