Bay St. Louis is located in southeastern Hancock County at 30°18'53" North, 89°20'39" West (30.314696, −89.344096).
It is situated on west side of the Bay of St. Louis, which empties into the Mississippi Sound.
The city is bordered to the north by the Jourdan River, the primary inlet of the bay.
The eastern border of the city is the Harrison County line in the middle of the bay.

According to the United States Census Bureau, the city has a total area of 26.6 square miles (69.0 km²), of which 14.7 square miles (38.1 km²) are land and 11.9 square miles (30.9 km²), or 44,82%, are water.
Bay St. Louis is located in southeastern Hancock County at 30°18'53" North, 89°20'39" West (30.314696, −89.344096). It is situated on west side of the Bay of St. Louis, which empties into the Mississippi Sound. The city is bordered to the north by the Jourdan River, the primary inlet of the bay. The eastern border of the city is the Harrison County line in the middle of the bay.
According to the United States Census Bureau, the city has a total area of 26.6 square miles (69.0 km²), of which 14.7 square miles (38.1 km²) are land and 11.9 square miles (30.9 km²), or 44,82%, are water.
Bay St. Louis, Mississippi