This study aims to explain the nature and legal status of qawāʿid fiqhiyyah according to Islamic legal scholars and to analyze their relevance and functional transformation in Indonesian marriage regulations, particularly the Compilation of Islamic Law. This library-based study employs a qualitative approach, drawing on classical and contemporary books, legal texts, and scholarly works concerning fiqh legal maxims, Islamic legal theory, jurisprudence, and marriage law. Data were collected through systematic literature study and analyzed using content analysis and descriptive analysis. The findings indicate that fiqh legal maxims constitute generalized principles formulated through jurists’ extensive reasoning concerning recurring fiqh issues and generally possess a ẓannī status. Their evidentiary position encompasses complementary and, according to some scholarly views, independent functions; nevertheless, the majority of scholars recognize them primarily as complementary evidence in legal determination. The study further identifies substantial relevance between fiqh legal maxims and Indonesian marriage regulations, particularly the Compilation of Islamic Law, across ten areas, including marriage registration, mahr, marriage agreements, ta‘līq al-ṭalāq, marital rights and obligations, marriage prohibitions, and ‘iddah. The study concludes that the movement from fiqh to national law reflects a functional transformation whereby legal maxims provide interpretive and complementary principles within a codified marriage-law framework. Academically, this study contributes to understanding how classical Islamic legal reasoning is adapted and incorporated into contemporary Indonesian marriage law.