The Old Cahokia Courthouse is a reconstructed French Colonial structure at 107 Elm Street, now managed as a historic site by the Illinois Historic Preservation Agency. The original building was constructed around 1740 in the poteaux-sur-sol ("posts on sill") style — a vertical log construction placed on a horizontal timber sill rather than directly in the ground, which proved more durable. It is the oldest surviving courthouse structure in the United States.
The building was moved, dismantled, and exhibited at the 1904 World's Fair in St. Louis before being returned to Cahokia and rebuilt on or near its original site. The reconstruction used original timbers where possible. The site is modest but historically significant: it marks the point where French colonial governance gave way to American territorial administration in the Illinois Country. Operating hours are limited; call ahead.