The bay's Vietnamese and Chinese names – Vịnh Bắc Bộ and Běibù Wān, respectively – both mean "Northern Bay". The name Tonkin, written "東京" in Hán-Nôm characters and Đông Kinh in the Vietnamese alphabet, means "eastern capital", and is the former toponym for Hanoi, the present capital of Vietnam. It should not to be confused with Tokyo, which is also written "東京" and also means "eastern capital". During Vietnam's French colonial era, the northern region was called Tonkin.
On 2 August 1964, United States President Lyndon B. Johnson claimed that North Vietnamese forces had twice attacked American destroyers in the Gulf of Tonkin. Known today as the Gulf of Tonkin Incident, this event spawned the Gulf of Tonkin Resolution of 7 August 1964, ultimately leading to open war between North Vietnam and the United States. It furthermore foreshadowed the major escalation of the Vietnam War in South Vietnam, which began with the landing of US regular combat troops at Da Nang in 1965.