Tonkin, also spelled Tongkin or Tongking, is the northernmost part of Vietnam, south of China's Yunnan and Guangxi Provinces, east of northern Laos, and west of the Gulf of Tonkin. Locally, it is known as Bắc Bộ, meaning "Northern Boundary". Located on the fertile delta of the Red River, Tonkin is rich in rice production.

The term derives from Đông Kinh (東京), the original name of Hanoi, which was the capital of Vietnam since the 7th century. (The name means "eastern capital", and is identical in meaning and written form in Chinese characters to that of Tokyo.)

France assumed sovereignty over Annam and Tonkin after the Franco-Chinese War (1884-1885). The French used the name of the capital for the entire region under its jurisdiction. The third part of Vietnam was Cochin China at the south.

Tonkinensis is subspecies of the dogwood plant cornus hongkongensis which can be found in Tonkin.