San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean. Specifically, both rivers flow into Suisun Bay, which flows through the Carquinez Strait to meet with the Napa River at the entrance to San Pablo Bay, which connects at its south end to San Francisco Bay. However, the entire group of interconnected bays is often referred to as “San Francisco Bay”.

San Francisco Bay is located in the U.S. state of California, surrounded by a contiguous region known as the San Francisco Bay Area (often simply "the Bay Area"), dominated by the large cities San Francisco, Oakland, and San Jose. The waterway entrance to San Francisco Bay from the Pacific Ocean is called the Golden Gate. Across the strait spans the Golden Gate Bridge.