Baba Sundar Das was an influential parcharak (preacher) of Sikhism.
Bakshi Singh 'Adil' in his Sampooran Ramgarhia Itihaas (lit. Comprehensive Ramgarhia History) states that Bhai Sundar Das hailed from Musapur Village in Punjab and met by Guru Nanak, the first Guru of the Sikhs, to later become an influential parcharak (preacher) of Sikhism.
A Gurdwara stands at the site of his historic samaadh in Musapur village today. He is believed to have been the founding father of the Sian (also spelt Seyan) gotra of Tarkhan Sikhs. Local lore indicates that he was given a 'baksish' by Guru Nanak Dev.
Genealogical accounts currently held at Musapur village indicate him having four sons of which one remained at Musapur and other moved and settled elsewhere across Punjab and India, giving rise to the four main Sian lineages that exist today.