St. Mary Roman Catholic Church, formally the Church of the Immaculate Conception of the Blessed Virgin Mary, is in the third oldest Roman Catholic parish in Detroit, Michigan. Designed by German-born Peter Dederichs and built for the formerly ethnic German parish of the 19th century, it is located at 646 Monroe Street in what is now considered the heart of the Greektown Historic District in downtown Detroit. It is often called "Old St. Mary's Church" to avoid confusion with other St. Mary's parishes: in the Redford neighborhood of Detroit, or in nearby Royal Oak, Monroe, or Wayne.