This study explores novice principals’ professional confidence within the context of Sweden’s National Principal Training Programme (NPTP). The significance of professional confidence, an area that has been relatively underexplored within educational leadership research, is elemental for fostering effective school leadership. This is particularly pertinent during the transitional phase from instructional to administrative positions. Using a qualitative case study design, data were collected from 256 novice principals via programme evaluations, including digital questionnaires and group discussions. Thematic analysis identified four key themes: building deeper knowledge, developing leadership capabilities, professional learning with peers, and role clarity. The findings highlight the transformative potential of the NPTP in fostering professional confidence through theoretical grounding, practical application, and peer collaboration. Principals reported increased confidence in school law, communication, management, and decision-making, linking these capabilities to leadership efficacy. Peer interactions and mentorship significantly contributed to professional growth, reinforcing identity and confidence. The study underscores the NPTP’s role as a model for context-sensitive leadership development, emphasising the importance of integrating knowledge, practice, and reflective learning in leadership preparation programmes. These findings have implications for designing leadership training that addresses the cognitive, relational, and contextual dimensions of professional development for school leaders.