The Gulf of Tonkin is a gulf at the northwestern portion of the South China Sea, located off the coasts of Tonkin (northern Vietnam) and South China. It has a total surface area of 126,250 km (48,750 sq mi). It is defined in the west and northwest by the northern coastline of Vietnam down to the Cồn Cỏ district, 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.

The name Tonkin, written "東京" in chữ Hán characters and Đông Kinh in the Vietnamese alphabet, means "eastern capital", and is the former toponym for Hanoi, the present capital of Vietnam. It is not to be confused with Tokyo, which is also written "東京" and also means "eastern capital". During the French colonial era, the northern region of today’s Vietnam was called Tonkin.

Bắc Bộ is the native Vietnamese name of Tonkin. The bay's Vietnamese and Chinese names – Vịnh Bắc Bộ and Běibù Wān, respectively – both mean "Northern Bay".

The Gulf of Tonkin is a relatively shallow portion of the Pacific Ocean; the majority of the gulf's ocean floor is less than 75 metres (246 ft) in depth, and no part of the gulf is submerged in more than 100 metres (330 ft) of water.

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. 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 South Vietnam. 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.

On December 25, 2000, Vietnam and China signed an Agreement on the Delimitation of the Gulf of Tonkin. An Agreement took effect on June 30, 2004, officially defining the maritime border between the two countries in the Gulf of Tonkin.

On March 1, 2024, China issued a “Declaration on the baselines of the territorial waters in the northern part of the Gulf of Tonkin”. Vietnam calls on China to respect international law.

• Port of Beibu Gulf
• Geography of China
• Geography of Vietnam
• Tonkin Gulf Yacht Club

• Cooke, Nola; Li, Tana; Anderson, James A., eds. (2011). The Tongking Gulf Through History. Philadelphia, PA: University of Pennsylvania Press. ISBN 978-0812243369.
• Churchman, Catherine (2016). The People Between the Rivers: The Rise and Fall of a Bronze Drum Culture, 200–750 CE. Lanham, MD: Rowman & Littlefield. ISBN 978-1442258600.

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