The principle of legality is one of the most fundamental principles of criminal law that plays a pivotal role in ensuring justice, transparency, and legitimacy of judicial actions. This principle emphasizes the need for legal prediction of crime and punishment and prevents the application of arbitrary punishments and broad interpretation of criminal laws. The importance of this principle in protecting the rights of the accused, ensuring legal security, and regulating criminal policy makes it necessary to examine its position in Iranian law and the international criminal system. The purpose of this research is to analyze the position and function of the principle of legality in Iranian criminal law and the international criminal system, to examine its impact on the rights of the accused and the legitimacy of the courts, and to assess the degree of coordination of domestic laws with international standards. The present research was conducted with a descriptive-analytical approach and using the library research method. The data were collected and analyzed through the study of domestic and international legal sources as well as interdisciplinary scientific articles. The research findings show that observing the principle of legality creates a clear and predictable framework for criminalization and punishment and prevents judicial arbitrariness. The comparative study indicates that although Iranian law has taken important steps in realizing this principle by emphasizing legal text and fair trial, in some cases it faces legislative and interpretative ambiguities. In contrast, the international criminal system, by providing clearer criteria, provides broader protection for the rights of the accused and imposes more effective restrictions on criminal prosecution. The research results indicate that strengthening the principle of legality requires continuous revision of domestic criminal laws, coordination with the standards of the international criminal system, and the development of supervisory and preventive mechanisms. Achieving this can, while enhancing legal security and public trust in the criminal justice system, provide the basis for more effective guarantee of the rights of defendants and increase the legitimacy of courts at the national and international levels.