The Ngạn people are a small local population of Tai-language speakers now living in the eastern districts of Cao Bằng province in northern Vietnam. They are said to be descendants of mercenary soldiers hired by the Mạc royal court during the 17^th century. It is the aim of this article to investigate where they came from, using Vietnamese and Chinese ethnological studies, on-site fieldwork and analysis of song texts, supplemented by information from a ritual text and a family register. My conclusion is that the original homeland of the Ngạn was in the Youjiang River valley in west-central Guangxi. Numerous strands of evidence point to a strong connection with the native chieftaincy of Tianzhou and the neighbouring chieftaincy of Si’en.