It is defined in the west and northwest by the northern coastline of Vietnam down to the Cồn Cỏ district,[1] in the north by China's Guangxi Zhuang Autonomous Region, and to the east by the Leizhou Peninsula and Hainan Island.

English sources from the People's Republic of China refer to the Gulf of Tonkin as Beibu Wan.

On 4 August 1964, United States President Lyndon B. Johnson claimed that North Vietnamese forces had twice attacked American destroyers in the Gulf of Tonkin.

[1][6] On March 1, 2024, China issued a “Declaration on the baselines of the territorial waters in the northern part of the Gulf of Tonkin”.

[7][10] One year later, in February 2025, Vietnam also announced baseline for determining its territorial waters width in the Gulf of Tonkin.