The concepts of moral weakness (ḍaʿf al-nafs) and non-rational adulthood (ghayr-rashīd) are among the fundamental notions in Islamic jurisprudence (Fiqh al-Imāmiyyah) and Iranian criminal law. Both concepts are directly related to the legal capacity (ahliyya), civil competence, and criminal responsibility of individuals. This research, using a descriptive-analytical and comparative method, examines the theoretical foundations, legal implications, and the conceptual relationship between “moral weakness” and “non-rationality” in Imāmiyyah jurisprudence and Article 596 of the Islamic Penal Code of Iran.In Shi‘a jurisprudence, moral weakness refers to a psychological or spiritual condition that arises from the dominance of passions or emotional fragility, resulting in the reduction of one’s rational willpower. It bears moral consequences but does not, by itself, nullify legal competence unless it reaches the level of safāhah (folly) or junūn (insanity). Conversely, non-rational adulthood signifies an individual’s inability to manage financial and personal affairs wisely, corresponding to the classical notion of safīh, which leads to partial or complete legal incapacitation.The comparative analysis reveals that while both concepts pertain to diminished capacity, they differ in nature, scope, and legal ramifications. Under Article 596 of the Islamic Penal Code (2013, amended 2019), the Iranian legislator incorporates these two notions together to criminalize acts of exploitation against persons suffering from psychological or cognitive vulnerability. This dual approach reflects a synthesis of Imāmiyyah jurisprudential principles—such as the no harm rule (lā ḍarar wa lā ḍirār), the principle of divine ownership (nafs wa māl al-muslim muḥtaram)—and modern protective criminal policies.Findings indicate that a clear conceptual distinction between moral weakness and non-rationality can enhance the precision of legal interpretations, improve judicial consistency, and strengthen protective mechanisms for vulnerable individuals. It further underscores the capability of Islamic jurisprudence to serve as a dynamic and ethical foundation for modern criminal justice frameworks in Iran.