Cochinchina or Cochin-China (, UK also ; Vietnamese: Đàng Trong (17th century - 18th century, Việt Nam (1802-1831), Đại Nam (1831-1862), Nam Kỳ (1862-1945); Khmer: កូសាំងស៊ីន, romanized: Kosăngsin; French: Cochinchine; Chinese: 交趾支那; pinyin: Jiāozhǐ zhīnà) is a historical exonym for part of Vietnam, depending on the contexts. Sometimes it referred to the whole of Vietnam, but it was commonly used to refer to the region south of the Gianh River.

In the 17th and 18th centuries, Vietnam was divided between the Trịnh lords to the north and the Nguyễn lords to the south. The two domains bordered each other on the Son–Gianh River. The northern section was called Tonkin by Europeans, and the southern part, Đàng Trong, was called Cochinchina by most Europeans and Quinam by the Dutch.

Lower Cochinchina (Basse-Cochinchine), whose principal city is Saigon, is the newest territory of the Vietnamese people in the movement of Nam tiến (Southward expansion). This region was also the first part of Vietnam to be colonized by the French. Inaugurated as the French Cochinchina in 1862, this colonial administrative unit reached its full extent from 1867 and was a constituent territory of French Indochina from 1887 until early 1945. So during the French colonial period, the label Cochinchina moved further south, and came to refer exclusively to the southernmost part of Vietnam. Beside the French colony of Cochinchina, the two other parts of Vietnam at the time were the French protectorates of Annam (Central Vietnam) and Tonkin (Northern Vietnam). South Vietnam (also called Nam Việt) was reorganized from the State of Vietnam after the Geneva Conference in 1954 by combining Lower Cochinchina with the southern part of Annam, the former protectorate.