The Roman Catholic Diocese of Sioux City (Latin: Diœcesis Siopolitanensis) is the Roman Catholic diocese for the northwestern quarter of the US state of Iowa. The diocese comprises 24 counties in northwestern Iowa, and it covers an area of 14,518 square miles (37,600 km2). The See city for the diocese is Sioux City. It is a suffragan see of the Archdiocese of Dubuque. The cathedral parish for this diocese is the Epiphany. R. Walker Nickless was ordained as bishop of Sioux City on 20 January 2006.