The Japanese invasion of French Indochina (, Futsu-in shinch) was a short undeclared military confrontation between Japan and France in northern French Indochina. Fighting lasted from 22 to 26 September 1940; the same time as the Battle of South Guangxi in the Sino-Japanese War, which was the main objective as to why Japan occupied Vietnam during this time.

The main objective of the Japanese was to prevent China from importing arms and fuel through French Indochina along the KunmingHai Phong Railway, from the Indochinese port of Haiphong, through the capital of Hanoi to the Chinese city of Kunming in Yunnan.Although an agreement had been reached between the French and Japanese governments prior to the outbreak of fighting, authorities were unable to control events on the ground for several days before the troops stood down. As per the prior agreement, Japan was allowed to occupy Tonkin in northern Indochina, and thus effectively blockade China.

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.