St. Mary Roman Catholic Church, formally the Church of the Immaculate Conception of the Blessed Virgin Mary is the third oldest parish in Detroit, Michigan. It is located at 646 Monroe Street in heart of Greektown Historic District within Detroit's downtown area. 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 nearby Royal Oak, Monroe, or Wayne.
The complex originally consisted of the church, rectory, constructed in 1876, school, constructed in 1868, and convent, completed in 1922. The convent was demolished in the early 2000s and replaced with a community center designed to resemble the 1841 church building.