The prohibition against same-clan marriage in Karo society is rooted in a genealogical kinship system that regulates marital boundaries to preserve identity, social order, and the continuity of clan relations. Issues arise when this customary norm is juxtaposed with Surah al-Nisā’ (4:23), which outlines categories of women forbidden for marriage under Islamic law. This study aims to analyze the construction of the same-clan marriage prohibition, examine the categories of women forbidden for marriage according to Tafsir al-Azhar, determine the status of the same-clan marriage prohibition within Hamka’s interpretation, and analyze the relationship between Qur’anic norms and Karo custom. The study employs a qualitative method based on library research, utilizing contextual-exegetical and socio-cultural approaches. Primary data consists of the Qur’an and Tafsir al-Azhar, while secondary data is drawn from literature on Islamic law and Karo society; data collection was conducted through documentation, followed by analysis using content analysis and comparative-contextual methods. The results indicate that the prohibition against same-clan marriage rests on the genealogical structure and kinship system of the Karo people. In contrast, Tafsir al-Azhar interprets the marital prohibitions in verse 23 based on ties of lineage (nasab), fosterage (radaʻah), and affinity (muṣāharah). Sharing the same clan cannot automatically be equated with being a maḥram based on lineage, as the clan represents a genealogical-customary identity with social functions. In contrast, lineage is a legal category carrying specific normative consequences. Therefore, customary legitimacy must be distinguished from Quranic legitimacy without forcing the text to serve as the direct basis for the same-clan marriage prohibition. This prohibition functions as a customary norm with social and genealogical legitimacy, while also highlighting the need for a balanced interpretation regarding the relationship between the Qur’an, Tafsir al-Azhar, and customary law.