The principle of criminal res judicata, as one of the fundamental guarantees of a fair trial, faces significant ambiguities in Iranian law concerning its conditions of realization and the criteria governing its application. The silence of the criminal legislator in precisely defining this doctrine has resulted in divergent interpretations among legal scholars and within judicial practice. Some jurists, drawing upon civil procedure and the three-fold requirements set forth in Article 84(6) of the Code of Civil Procedure, regard the unity of parties as an essential condition for the establishment of res judicata in criminal proceedings. In contrast, the opposing view emphasizes the centrality of the “unity of the criminal act” and its decisive role in preventing parallel proceedings. This divergence has likewise manifested itself in judicial practice, leading to inconsistent and contradictory judgments. This study, employing a descriptive–analytical methodology and relying on library-based sources, domestic legislation, Islamic jurisprudential foundations, and international instruments, examines the role of the unity-of-complainant requirement in the realization of criminal res judicata. In this regard, while briefly referring to sample criminal judgments illustrating the tension between the two interpretive approaches, the study analyzes the theoretical and legislative foundations of the doctrine and evaluates the practical consequences associated with each perspective. The findings indicate that recognizing unity of complainant as a necessary condition, in practice, destabilizes the finality of judgments, enables the filing of multiple complaints concerning the same factual episode, and reduces the overall efficiency of the criminal justice system. Conversely, adopting the criteria of “unity of the criminal act” and “unity of the accused” not only aligns with criminal law principles and procedural norms but also proves more effective in preventing contradictory judgments and enhancing coherence within judicial practice.