Customary marriage remains a living institution among the Sasak community of Gadung Mas Village, West Sakra Subdistrict, East Lombok Regency, West Nusa Tenggara, even where such marriages are never registered with the state. This study examines how customary legitimacy and state legal recognition interact when an unregistered customary marriage later generates an inheritance dispute. Using empirical legal research with a socio-legal approach, data were collected through in-depth interviews with a customary leader, a religious leader, and the village head of Gadung Mas, supplemented by a systematic review of national legislation and Scopus-indexed comparative literature on legal pluralism and Indonesian inheritance law. The findings show that marital legitimacy in Gadung Mas rests on three interlocking pillars: adat procession as social legitimacy, religious solemnization (akad nikah) as normative validity, and state registration as administrative certainty; the third pillar is frequently deferred or omitted without the marriage being regarded, locally, as defective. Inheritance disputes are resolved through a multi-tier mechanism that begins with family deliberation (musyawarah keluarga), proceeds to mediation by a customary leader (tokoh adat) working alongside a religious figure, and escalates to the Religious Court only when consensus fails. Read through Griffiths' theory of legal pluralism, Ehrlich's (1936) and Soekanto's (1981) concept of living law, and Radbruch's triad of legal purposes, this pattern is best understood not as a competition between adat and state law but as a functional division of labor: adat law supplies social legitimacy and restorative resolution, while state law supplies evidentiary certainty when adat consensus is unattainable. The study's original contribution is a Social–Juridical Legitimacy Harmonization (SJLH) model that maps how these two forms of legitimacy combine across the marriage cycle and the stages of dispute resolution.