St. Thomas Orthodox Church in Sioux City, Iowa is a parish of the Chicago and Middle American Deanery of the Diocese of Chicago and Middle-America, part of the Russian Orthodox Church Outside of Russia, a constituent archdiocese of the Russian Orthodox Church. Founded in 1916, as part of the Greek Orthodox Patriarchate of Antioch to serve the Syrian and Lebanese immigrant community that existed in Sioux City at that time, it is the oldest Orthodox parish in Sioux City and has become a pan-Orthodox community with members from across the Siouxland region.