The Old Bay St. Louis Historic District is a historic district in the small coastal town of Bay St. Louis, Mississippi, US. The district is composed of and contains 939 resources, including 681 contributing properties. In 1980, the National Park Service had certified the boundaries of four historic districts in the city. The earlier four historic district were named the Beach Boulevard Historic District, Washington Street Historic District, Sycamore Street Historic District, and Main Street Historic District. The city's districts were reassessed by the Federal Emergency Management Agency (FEMA) in the aftermath of Hurricane Katrina on August 29, 2005 and reclassified as the Old Bay St. Louis Historic District. It has been listed by the National Register of Historic Places since July 8, 2010.
The largest of the four distinct districts is the Beach Boulevard Historic District, which encompassed 690 buildings and and includes a nearly two-mile strip of Beach Boulevard, as well as a business core and residential neighborhood. The Old Bay St. Louis Historic District includes the extant sections of the 1980s defined Beach Boulevard Historic District, as well as a large residential area to the west of the old district boundaries.
Around 30% of the buildings in Beach Boulevard Historic District were destroyed after Hurricane Katrina; and many were lost in the earlier Hurricane Camille in 1969. Those houses that survive were built mostly between 1860 and 1960, and they include Central-passage houses, Queen Anne, Colonial Revival, Spanish Revival, and Craftsman style houses.
The Washington Street Historic District is a , twelve-building district located primarily on the south side of Washington Street.
The Sycamore Street Historic District is composed of buildings mostly on the north side of Sycamore Street, and to the east and west of Old Spanish Trail. Much of the historic resources listed in the Sycamore Street Historic District in 1980 were destroyed after Hurricane Katrina.
The Main Street Historic District is and six buildings located on the south side of Main Street, near U.S. Highway 90.