Mentoring is one of the professional development tools supporting school principals. Thanks to mentoring, principals can overcome the problems they confront and meet the expectations of different stakeholders. Thus, principals are able to perform more effectively in schools. Mentoring has become a formal process for principals in various countries for a long time. Moreover, several researchers from different countries focus on the various impacts of mentoring on principals. One of these is the self-efficacy of principals. Mentoring has the potential to enhance the principals' self-efficacy. However, it is difficult to note that mentoring and its influence on principals' self-efficacy have been analyzed thoroughly. In this context, this phenomenological study aims to overcome the current inadequacy. The study data was collected based on criterion and snowball sampling from eight principals working in Kahramanmaraş province. MAXQDA 2020 was utilized for data analysis. The results revealed that mentoring plays a critical role in the self-efficacy beliefs of principals. In other words, it develops principals' managerial, instructional, and ethical competencies. This finding significantly contributes to the literature on principals' professional development, professional learning, and self-efficacy. Several suggestions have been offered for policymakers and researchers about the formalization of mentoring and the details of its implementation.