Natchesium or Natchez is a Roman Catholic Diocese. As it is a titular see, it is not an active diocese. The diocese was founded on July 28, 1837 and originally was the Catholic diocese for the entire state of Mississippi. Even though the diocese was officially created in 1837, the first bishop of the diocese, John Mary Chanche, S.S. (1795–1852), was not appointed until 1840. He did not arrive in Natchez until May 1841.
Over the years, as populations within the state shifted, it became obvious that the operations of the diocese should be moved to Jackson, the state capital. On December 18, 1956, the diocese was renamed the Diocese of Natchez-Jackson and many operations of the diocese moved to Jackson. On March 1, 1977, the Mississippi diocese was split into two dioceses: the Diocese of Jackson and the Diocese of Biloxi. After this, the Diocese of Natchez became a titular see.