Background: Waqf plays significant religious and social roles in Indonesia, including supporting religious, educational, and community facilities. However, waqf buildings established on uncertified customary land encounter legal uncertainty due to differences between customary law, Islamic law, and the national land registration system. Objective: This study examines the juridical status of waqf buildings established on customary land that has not been formally registered or certified under Indonesian national land law. Methods: This study applies normative legal research using a statutory approach. Legal materials were obtained through library research, including legislation, legal doctrines, academic literature, and regulatory documents. The data were analyzed qualitatively through legal interpretation, systematization, and evaluation of legal norms related to waqf and uncertified customary land. Results: The findings reveal legal dualism arising from the interaction of Islamic law, customary law, and positive law. Although waqf may be recognized as valid from religious and customary perspectives, the absence of formal registration and certification limits its legal recognition and protection within the national legal system. This situation creates uncertainty regarding the management, protection, and sustainability of waqf assets and increases the risk of disputes and competing claims. Conclusion: Harmonization between customary law and national land law is necessary to strengthen legal certainty for waqf assets on uncertified customary land. The Isbat Waqf mechanism through the Religious Court offers a potential solution to recognize undocumented historical waqf and improve legal protection and governance of waqf in Indonesia.