Purpose Empirical studies show that principals' identity and development are essential for successful leadership and positively influencing school results. Empirical arguments exist for continued research into principals' identities, as they can contribute to leadership development. Therefore, this study explored how principals' peer mentoring during training can help shape their leadership identity. The purpose of this study was to explore novice principals' professional socialisation and the shaping of leadership identity in Sweden during their participation in the mandatory principal programme. Design/methodology/approach This study used qualitative interviews with 35 novice principals participating in peer mentoring for professional development in Sweden's National Principal Training Programme (NPTP). Data were analysed using inductive analysis and a deductive approach based on a theoretical model of principalship.FindingsThe findings showed that novice principals describe peer mentoring as essential to the NPTP. Peer mentoring provides opportunities for sharing and discussing critical professional issues and wicked problems as a source of professional development and strengthens leadership identity. Practical implications The study's findings can be helpful for leadership preparation programmes globally and deepen the knowledge base on professional socialisation and the development of leadership identity. Originality/value The study's findings are valuable as they contribute from the Scandinavian context and add to the current knowledge base on professional socialisation and leadership identity in leadership training programmes for novice principals. Further, Saarukkas's (2017) model has been useful in illustrating these findings theoretically, extending the use of the model.