Abstarct: This research aimed to investigate the impact of the three dimensions of organizational commitment (affective, continuance, and normative) on job performance, considering the mediating role of organizational citizenship behavior among employees of the Tehran Province Water and Wastewater Company. The statistical population of this study consisted of all employees of this company, approximately 3500 individuals, from which 380 people were selected as the sample using Morgan's table and simple random sampling. Data were collected using standard questionnaires and analyzed using appropriate statistical methods. Findings from hypothesis testing indicated that all three dimensions of organizational commitment (affective, continuance, and normative) have a positive and significant impact on both organizational citizenship behavior and job performance. Furthermore, the positive and significant relationship between organizational citizenship behavior and job performance was also confirmed. On the other hand, results related to the mediating role showed that organizational citizenship behavior plays a partial mediating role in the relationship between affective commitment and continuance commitment with job performance. This finding suggests that part of the positive impact of affective and continuance commitment on job performance is realized through the reinforcement and encouragement of employees' extra-role citizenship behaviors. These results align with the perspectives of many previous researchers who have emphasized the profound lixnk between attitudinal capital (such as commitment) and behavioral capital (such as citizenship behavior), ultimately leading to desirable operational outcomes (such as high performance). The significance of these findings lies in demonstrating that to enhance job performance in public service organizations like the Water and Wastewater Company, focusing solely on formal and technical requirements is insufficient. Rather, strengthening feelings of belonging, a sense of moral responsibility, and creating exchange-baxsed bonds with employees can pave the way for the voluntary emergence of beneficial behaviors beyond job descxriptions, which in turn provides a strong foundation for improving overall performance.