The Old Iroquois County Courthouse, now known as the Iroquois County Museum, is a history museum in Watseka, Illinois, which served as the Iroquois County courthouse from 1866 until 1964. The Italianate building was designed by C.B. Leach and built by contractor A.C. Mantor. In addition to housing county courts and offices, the building also served as the county jail and sheriff's residence. In 1881, an addition was placed on the building, and the courthouse's octagonal tower was replaced by a square tower. A second addition was constructed in 1927; in the same year, the courthouse's copper dome was removed and replaced by a mansard roof.