‌Context & Objective: The institution of marriage (Nikāḥ) is fundamentally designed by the Islamic Lawgiver (Shāri‘) to achieve tranquility for the spouses. Although Islamic jurisprudence (Fiqh) permits a husband to simultaneously contract up to four permanent marriages, the subsequent marriage of the husband, while the first marital bond subsists, is conventionally understood in contemporary societies to cause distress to the first wife, thereby undermining this foundational goal. To safeguard this marital objective, it is a frequent practice in Iran that the wife includes an ancillary stipulation requiring the husband to abstain from taking another spouse. The research acknowledges a vacuum in comprehensive legal scholarship that offers a principled and existentially focused solution consistent with the national legal framework. Therefore, the central research purpose is to identify the necessary consequential legal effect of this specific stipulation on the legal status of the husband’s subsequent marriage. The primary research question is: Assuming the validity of the condition, what is the legal status of the subsequent marriage when the husband commits a breach? The authors proceed with the hypothesis that, upon the breach of the stipulation of abstinence from remarriage, the husband’s subsequent marriage is subject to nullity (Buṭlān). The discussion outlines a three-part structure: first, validating the stipulation; second, analyzing the competing theories regarding the legal guarantee of the breach; and third, justifying the selection of the nullity theory based on core criteria.
 ‌Method & Approach: This study employs a doctrinal methodology coupled with a descriptive-analytical approach, drawing upon authoritative library resources in Islamic jurisprudence and Iranian law. As a necessary prerequisite, the study first addressed the question of the stipulation's validity (Siḥḥat). Arguments claiming the stipulation is void—such as those asserting it constitutes the prohibition of a permissible act (Taḥrīm Halāl) or represents the outright forfeiture of the right to marry (prohibited by Iranian Civil Code Article 959)—were systematically evaluated and rebutted as being legally insufficient. Accordingly, the validity of the condition was established and affirmed in light of the presumption of validity of stipulations (Aṣalat al-Siḥḥa). Following this determination, the analytical phase critically examined three principal jurisprudential theories concerning the consequential effect of the breach on the subsequent marriage: the Theory of Validity, the Theory of Ineffectuality (Adam-e Nufūdh), and the Theory of Nullity.
 ‌Findings: The primary finding validates the hypothesis: the husband’s subsequent marriage, contracted in violation of the stipulation of abstinence, is subject to nullity (Buṭlān). This conclusion rests upon the convergence of two foundational criteria. The first criterion is the analysis of the common intent of the stipulating parties. The study posits that the conventional understanding (‘Urf) and the true intention of the wife in demanding abstinence is to prevent the husband’s subsequent marriage from ever gaining legal existence in the realm of consideration. Therefore, although structured as a stipulation of abstaining from a juridical act, its actual effect is equivalent to a negative Condition of Outcome, resulting in the Waiver of the Right to Marry. Nullity is also independently supported by the jurisprudential principle of the forfeiture of legitimate capacity, whereby the husband is deemed to have forfeited the capacity to validly execute the conflicting legal act by virtue of the preceding condition. The second criterion is the conformity of the selected theory with Islamic jurisprudence and the Iranian legal system. By analyzing various provisions of the Civil Code (specifically Articles 454, 455, 474, and 679), the study extracts a general rule demonstrating that the Lawgiver treats the violation of a stipulation to abstain from a juridical act as resulting in the nullity of the subsequent conflicting legal act.
 ‌Conclusion: In summary, the stipulation requiring the husband to abstain from subsequent marriage is legally valid. The breach of this condition necessitates the adoption of the Theory of Nullity for the subsequent marriage, a result justified by the common intent of the parties and confirmed by analogous legal rules embedded within the Iranian Civil Code. While this result provides a robust and principled legal guarantee, the nullity of the marriage carries serious potential negative social ramifications, primarily concerning the instability of the second wife's legal status and the potential deprivation of rights for children born of the void marriage. To mitigate these consequences, the research recommends a set of necessary complementary protective measures. These include the explicit legislative codification of nullity as the definitive legal guarantee; imposing a duty on registrars to warn parties of the nullity consequence; implementing technical checks (e.g., electronic inquiry) to prevent the registration of a void subsequent marriage; establishing special legal protections and civil compensation for the second wife who married in good faith and ignorance; and resolving the issue of inheritance for children. To protect children while respecting theological rulings, the recommendation is to mandate that the natural father’s estate owes the child a civil debt equivalent to the legitimate share of inheritance, thereby preventing complete deprivation. Through these comprehensive civil and protective mechanisms, the Theory of Nullity can be established as the most principled and effective legal guarantee.