Annulment of marriage due to defect is one of the fundamental institutions of family law, whose origin lies in Imamiyyah jurisprudence, and which has been coherently regulated in the Iranian Civil Code through the adoption of jurisprudential foundations. This study, adopting a jurisprudential–legal approach, analyzes the concept of defect, its scope and instances, as well as the legal effects arising from the annulment of marriage due to defect within the Iranian legal system. The importance of this discussion stems from the fact that the existence of effective physical or psychological defects in one of the spouses may disrupt the philosophy and essential functions of the marriage contract—such as tranquility and affection, fulfillment of legitimate needs, procreation, and the preservation of the moral order of the family—and render the continuation of the contract burdensome and harmful for the other party; a situation that is incompatible with established jurisprudential principles, particularly the rule of la darar wa la dirar (no harm and no harassment). The present research has been conducted using a descriptive–analytical method and a comparative study of authoritative sources of Imamiyyah jurisprudence and the provisions of the Iranian Civil Code. The findings indicate that Imamiyyah jurisprudence defines defect as any deficiency or flaw that removes a person from the normal and sound state of creation and prevents the realization of the objectives of marriage. This foundation is directly reflected in Articles 1121 to 1123 of the Iranian Civil Code, whereby the legislator, by distinguishing between common defects, defects specific to the husband, and defects specific to the wife, has recognized the right of annulment of marriage for each party in specified cases. This approach demonstrates the legislator’s effort to establish legal balance between the spouses and to protect the injured party within the framework of Islamic and legal standards.