Tonkin, also spelled Tongkin, Tonquin or Tongking, is an exonym referring to the northern region of Vietnam. During the 17th and 18th centuries, this term referred to the domain Đàng Ngoài under Trịnh lords' control, including both the Northern and Thanh-Nghệ regions, north of the Gianh River. From 1884 to early 1945, this term was used for the French protectorate of Tonkin, composed of only the Northern region.

Tonkin, or Bắc Kỳ, was a French protectorate encompassing modern Northern Vietnam. Like the French protectorate of Annam, Tonkin was still nominally ruled by the Nguyễn dynasty, but in 1886, the French separated Tonkin from the Nguyễn imperial court in Huế by establishing the office of "Viceroy". However, on 26 July 1897, the position of Viceroy was abolished, officially making the French resident-superior of Tonkin both the representative of the French colonial administration and the Nguyễn dynasty court in Huế, giving him the power to appoint local mandarins. In 1887, Tonkin became a part of the Union of Indochina.