The Gulf of Tonkin, in Vietnamese: Vịnh Bắc Bộ or in Chinese: Beibu Wan is an arm of the South China Sea. Covering an area of 126,250 km², the gulf borders Vietnam on the northwest, west and southwest. The Chinese mainland lies to the north while the Island of Hainan forms the eastern limits of the gulf. The gulf is notably shallow (less than 60 meters deep). Haiphong, Vietnam and Beihai, China are the chief ports. Numerous small islands are located in the gulf, most of which are concentrated in the northwestern gulf. Of note are larger islands of Bach Long Vi and Cat Ba of Vietnam and Weizhou of China. The Red River is the main river flowing into the Gulf.

The name Tonkin, written 東京 in Chinese characters and Đông Kinh in Vietnamese, means 'Eastern Capital', and is the former toponym for Hanoi, the capital of Vietnam. (Incidentally, the same Chinese characters are used to write "Tokyo", capital of Japan.)

In August of 1964, United States President Lyndon B. Johnson incorrectly claimed that North Vietnamese forces had twice attacked American destroyers in the Gulf of Tonkin. Although there was a first attack, allegedly in response to U.S. equipped and orchestrated South Vietnam's commando raids on the coast, claims of a second attack were later proven to be unfounded. Known today as the Gulf of Tonkin Incident, this led to the open involvement of the United States in the Vietnam War, with the Gulf of Tonkin Resolution.