Reason, as one of the most prominent divine gifts, plays a pivotal role in achieving psychological well-being. Reason provides the ability to distinguish between good and evil and guides humans to make informed and logical decisions. The Holy Quran and the narrations of the infallible Imams (AS), by repeatedly emphasizing the high status of reason, have pointed out the necessity of utilizing it in order to understand the truths more deeply and achieve correct knowledge. The present study, using the descriptive-analytical research method, examines the principles governing reason and its role in psychological well-being from the perspective of Imam Ali (AS). In this regard, after analyzing concepts such as "reason" and "psychological well-being", the fundamental principles governing reason, including thinking, science, experience, and exploration, have been examined. In the following, the key roles of reason in achieving psychological well-being, such as truth-seeking, guidance, discovering the unknown, moderation, creating friendship, and self-management, have been analyzed. The findings of the study show that reason, as the most central factor in creating satisfaction and psychological well-being, plays a significant role in achieving this through cognitive and ethical mechanisms. This study emphasizes the importance of reason as a strategic tool in guiding individual and social behaviors and attitudes to achieve sustainable psychological well-being.