One of the important issues in the law of contract is the impact of third-party cooperation with the contractual promisor in breaching his obligations. In the legal systems of France and Belgium, third-party cooperation in the breach of contractual obligations has been considered a cause of civil liability. This research, using an analytical, logical, and comparative method, while explaining the basics, conditions, and effects of the aforesaid liability in the legal systems of France and Belgium, investigates the viewpoint of the legal system of Iran. Based on the results of this research, in French doctrine and precedent, joint and several liability of the promisor and the third party is the theory accepted for the compensation of damages caused to the promisee. Moreover, in cases where the third party concludes a contract with the promisor in order to breach his obligations, non-opposability or annulment of the aforesaid contracts has been proposed as the appropriate way to restore the previous situation. In the legal system of Iran, unenforceability of the aforementioned contracts is acceptable. It is also defensible to hold the promisor and the third party equally liable toward the contractual promisee for the resulting damages, unless their conduct efficacy is different.