The concept of maʿrūf is one of the fundamental principles of the Qur'an for regulating social relations and organizing the reciprocal rights and obligations of individuals. Despite its extensive use in Qur'anic verses concerning social, familial, and legal matters, scholars have proposed differing interpretations regarding its essential nature and its relationship with rational custom (ʿurf al-ʿuqalāʾ). The primary objective of this study is to clarify the nature of maʿrūf in the Qur'an's social verses and to examine the role of rational custom in shaping and identifying its practical manifestations. The research adopts a historical-exegetical approach combined with a custom-oriented linguistic analysis. Within this framework, the occurrences of the term maʿrūf across different Qur'anic contexts are identified and analyzed, and their semantic implications are examined through historical evidence, lexical data, and the socio-cultural context of the Qur'an's revelation. Furthermore, the relationship between maʿrūf and rational custom is explored through a comparative analysis of the historical context of pre-Islamic Arabia and the early Islamic community. The findings indicate that, in its social applications, maʿrūf generally refers to norms that were recognized and accepted within the rational customs of human societies. Divine revelation affirmed, refined, or supplemented these norms in accordance with overarching principles such as justice, human dignity, benevolence, and the rejection of oppression. The analysis further demonstrates that the Qur'an employs two principal approaches in its engagement with prevailing social customs: first, reforming acceptable customs by delimiting, qualifying, and reorienting them toward ethical values; and second, rejecting outright those customs that are incompatible with human dignity and the higher objectives of Islamic law (maqāṣid al-sharīʿah). The study concludes that while the Qur'an acknowledges the regulatory function of rational custom in governing social relations, it conditions the ultimate validity of social norms upon their conformity with the moral standards established by divine revelation. Accordingly, maʿrūf may be understood as a mediating concept between rational custom and revealed guidance, facilitating the realization of social justice and the establishment of a sound legal order. Finally, the article proposes a conceptual framework for reexamining contemporary Islamic jurisprudence in addressing emerging social customs.