Res judicata constitutes one of the fundamental principles underpinning the international arbitration system, serving to ensure the finality and stability of arbitral awards and to preclude the possibility of re-litigation. Nevertheless, a critical question arises: are there circumstances under which the full application of the effects of res judicata may be precluded or rendered impracticable? Adopting an analytical and comparative approach, and acknowledging the existing lacunae in international arbitration instruments as well as notable deficiencies in contemporary legal scholarship, this article aims to examine the exceptions to the doctrine of res judicata through an in-depth study of arbitral practice. Specifically, it explores instances in which arbitral tribunals have declined to give full effect to res judicata, thereby delineating the boundaries of this principle. The legal analysis—grounded in applicable rules and principles—leads to the conclusion that, notwithstanding the central role of res judicata in securing the efficiency and integrity of arbitral proceedings, international tribunals have, in exceptional cases, refrained from its strict application. These exceptions include, inter alia: fraud in the arbitral process, issuance of a corrective or supplemental award, conflict of the award with international public policy, emergence of new evidence, fundamental changes in the legal or factual matrix post-award, lack of jurisdiction, and violations of due process. Recognizing such exceptions, on the one hand, safeguards the efficiency of the arbitral process, while, on the other hand, it strengthens the legitimacy, enhances the flexibility, and fosters greater confidence in the international arbitration system.