The Roman Catholic Diocese of Saint Joseph was an ecclesiastical territory or diocese of the Roman Catholic Church in the northwestern part of the state of Missouri in the United States, erected on March 3, 1868, with territories taken from the Archdiocese of Saint Louis. Its first bishop was John Joseph Hogan. On July 2, 1956, the diocese lost territory to the newly erected Diocese of Jefferson City and the Diocese of Springfield–Cape Girardeau. On that date it was united to the Diocese of Kansas City, which was renamed the Diocese of Kansas City–Saint Joseph.