northwest arm of the South China Sea, bounded by China (north and east), Hainan Island (east), and northern Vietnam (west). The gulf is 300 miles (500 km) long, 150 miles (250 km) wide, and up to 230 feet (70 metres) deep. The main shipping route is via the Hainan Strait, between China and Hainan Island. The gulf receives the Red River, and its main ports include Ben Thuy and Haiphong in northern Vietnam and Beihai (Pakhoi) in China.

The report of North Vietnamese torpedo boats firing on two U.S. destroyers in 1964 resulted in the U.S. Congress adopting the Gulf of Tonkin Resolution to support increased U.S. troop involvement in the Vietnam War. In 1972 the gulf was the scene of increased naval activity when the U.S. Navy mined the entrances to North Vietnamese ports. Since the end of the Vietnam War, the Gulf of Tonkin has been the site of oil exploration by many multinational companies.