The Thomas Johnson Polygonal Barn is an historic structure located near Wellman in rural Washington County, Iowa, United States. The NRHP calls it the Thomas Jordan Polygonal Barn, but then attributes it to Thomas Johnson. It has been listed on the National Register of Historic Places since 1986. The 16-sided building measures 40 feet (12.2 m) around. The structure is covered in red vertical siding and is topped by a sectional conical roof, with windows on the south side and an aerator.