Old St. Mary's, Old St Mary's, or Old Saint Mary's may refer to:
Old St. Mary's Church (Cincinnati, Ohio), church in United States Old St Mary's, Walmer, church in Kent, England Old St. Mary's Church (Milwaukee, Wisconsin), church in United States Old St. Mary's Catholic Church (Fredericksburg, Texas), church in United States Old St Mary's Church, West Bergholt, church in Essex, England Old Saint Mary's Cathedral, cathedral in San Francisco, California, United States Old St. Mary's Church (Clonmel), church in Clonmel, Ireland St. Mary's Roman Catholic Church (Philadelphia), also known as Old St. Mary's