San Diego Bay is a natural harbor and deepwater port located in San Diego County, California near the U.S.–Mexico border. The bay, which is 12 miles (19 km) long and 1 to 3 miles wide, is the third largest of the three large, protected natural bays on California's 840 miles (1,350 km) of coastline, after San Francisco Bay and Humboldt Bay. The highly urbanized land adjacent to the bay includes the city of San Diego and four other cities: National City, Chula Vista, Imperial Beach and Coronado.

National City is a city located in the South Bay region of the San Diego metropolitan area, in southwestern San Diego County, California.