The present study aims to examine the humanization of criminal law and to assess the role of human dignity and the certainty of punishment in enhancing the effectiveness of rehabilitation and the legitimacy of criminal policy. The necessity of this research arises from the growing disjunction and erosion between the certainty of punishments and their actual enforcement within the criminal justice system, particularly in the context of severe penalties and alternatives to imprisonment. This trend results in violations of human dignity, the weakening of legitimacy, and a decline in the effectiveness of rehabilitation. Employing a descriptive–analytical–comparative approach and relying on library-based research, this article evaluates the criminal justice system of Iran alongside those of selected Western countries. The findings indicate that rehabilitation, as a psychological process rooted in learning theory, is not achieved through the severity of punishment; rather, it is contingent upon the certainty and consistency of reward and punishment. In fact, the adoption of severity-oriented policies and the unrealistic design of punishments, in addition to undermining the certainty of punishment, constitute a tangible and serious threat to human dignity and the rehabilitative function of criminal justice. In contrast, this study proposes the adoption of a model based on proportionate yet certain punishments, which not only safeguards human dignity but also enhances the effectiveness of genuine deterrence and strengthens the legitimacy of the criminal justice system. Furthermore, the present research emphasizes the fair and predictable enforcement of punishments as an effective and efficient strategy in contemporary criminal policy.