Surat Tumbaga Holing is a traditional legal manuscript of the Mandailing community in North Sumatra, functioning as a primary source of customary law. It outlines normative structures governing social life, kinship, and conflict resolution, reflecting deeply rooted cultural and moral values. This study explores its legal structure, mechanisms of inheritance, implementation, and its role in promoting justice and harmony. Using a normative juridical method combined with qualitative analysis, data were collected through document studies, interviews with raja adat, and observations of customary practices. Findings show that the Surat has a codified yet adaptable structure, transmitted through oral tradition and ritual. Its enforcement relies on moral obligation, social trust, and restorative practices rather than formal penalties. As a living document, it continues to serve as a symbol of identity and a practical tool for maintaining justice. This research affirms the significance of indigenous legal systems within the broader framework of legal pluralism.