This study aims to examine the development of wasiat wajibah (obligatory bequest) within Indonesian Islamic inheritance law, particularly its application to non-Muslim individuals who are precluded from inheriting due to religious differences, and to examine the juridical and methodological grounds underlying this development. This study employs a normative legal research method using statutory, case, conceptual, Usul al-Fiqh, and Maqāṣid al-Sharī‘ah approaches. The cases examined include Supreme Court Decisions Nos. 368 K/AG/1995, 51 K/AG/1999, 16 K/AG/2010, 218 K/AG/2016, and 331 K/AG/2018, which demonstrate the development of wasiat wajibah for non-Muslim children and spouses. The novelty of this study lies in formulating a methodological nexus between the prohibition of interfaith inheritance and the granting of wasiat wajibah by conceptually distinguishing irth/farā’iḍ (inheritance) from wasiyyah (bequest), analyzed through an integrated Usul al-Fiqh and Maqāṣid al-Sharī‘ah approach. The findings demonstrate that judicial precedents have expanded the application of Article 209 of the Compilation of Islamic Law (KHI), which textually applies to adopted children and adoptive parents, into an instrument for granting property to non-Muslim parties who do not have the legal status of heirs. Methodologically, such a grant is not construed as an inheritance right arising from an interfaith family relationship, but rather as a transfer of property through the institution of wasiyyah, and therefore does not directly negate the prohibition against interfaith inheritance. This study concludes that wasiat wajibah may serve as an instrument of substantive justice in cases of interfaith inheritance, provided that it does not alter the recipient’s status into that of an heir and that its application is constrained by a sound legal basis, proportionality in the amount granted, and defensible parameters derived from Usul al-Fiqh and Maqāṣid al-Sharī‘ah.