The customization of criminal law, which manifests itself across various stages including pre-legislation, legislation, interpretation, adjudication, and the execution of sentences, constitutes a methodology that delineates criminal norms within legal processes by drawing upon the prevailing customs of society and the cultural contexts of individuals. Numerous Islamic countries, notwithstanding their adherence to Islamic precepts and regulations, have prioritized customary practices in their norm-setting frameworks and judicial determinations. This article, employing a descriptive-analytical approach, seeks to address the following inquiries: How can the duality between custom and Sharia be reconciled within the legislative systems of nations governed by Islamic law? What imperatives necessitate the customization of criminal law? What are the specific manifestations of this customization within the Iranian context? What are the consequences arising from the customization of criminal law? The study examines the conceptualization of customizing criminal law, its expressions across different dimensions of criminal justice structures, and evaluates both its positive and negative implications. Research findings suggest that certain behaviors undergo criminalization or decriminalization over time as a result of normative transformations, thereby underscoring the profound influence of customs on criminal law. Customs, as reflections of societal culture and values, serve to address legal lacunae and enhance the social legitimacy of criminal law. Consequently, although the principle of legality in criminal law constrains the post-legislative role of customs, the integration of customs into the norm-setting phase and other criminal justice processes is deemed beneficial for religious, ethical, sociological, cultural, and criminological reasons. While acknowledging potential adverse outcomes, the overarching advantages—such as heightened legal legitimacy, improved alignment with societal norms, reinforced public order, reduced public discontent, and the prevention of governmental overreach—are considered to outweigh the disadvantages. Nevertheless, it is imperative to recognize that customs may occasionally conflict with Sharia or statutory law, potentially undermining legal authority and giving rise to theoretical and practical conflicts.