The Mandailing are an ethnic group in Sumatra, Indonesia that is commonly associated with the Batak people. They are found mainly in the northern section of the island of Sumatra in Indonesia. They came under the influence of the Kaum Padri who ruled the Minangkabau of Tanah Datar. As a result, the Mandailing were influenced by Muslim culture and converted to Islam. There are also a group of Mandailing in Malaysia, especially in the states of Selangor and Perak. They are closely related to the Angkola and Toba.

Batak is a collective term used to identify a number of closely related Austronesian ethnic groups predominantly found in North Sumatra, Indonesia, who speak Batak languages. The term is used to include the Karo, Pakpak, Simalungun, Toba, Angkola, and Mandailing, related ethnic groups with distinct languages and traditional customs (adat).