Background <p>Clinician-educators (CEs) play a critical role in improving the quality of healthcare by promoting the staff development (SD) among healthcare professionals. However, little is known about how clinical supervisors transition into CEs through their contribution to SD.</p> Methods <p>We employed a constructionist approach to thematic analysis to explore the professional identity formation (PIF) of supervisors as they evolved into CEs by contributing to an SD course on neonatal cardiopulmonary resuscitation. Semi-structured interviews were conducted with nineteen Japanese neonatology CEs. Using the socialisation factors of PIF as the theoretical framework, we generated themes influencing their identity formation as CEs.</p> Results <p>Prior to contributing to SD, neonatology supervisors had only a vague sense of responsibility and leadership within their clinical setting. Their educational approach was traditional and unidirectional, and they experienced mixed feelings – such as honour and anxiety – when invited to participate in SD as CEs. After contributing to SD, they developed their professional identity as CEs through rich socialisation factors, including role models, educational experiences, peer CEs, and independence from peer practitioners, aligning with the socialisation factors of PIF. This transformation was marked by a strong desire to pass on their mission to the next generation of healthcare professionals while contributing to society, motivation for educational activities with positive emotions, and the adoption of a learner-centred, bidirectional approach to education.</p> Conclusions <p>This study elucidates the process by which neonatology supervisors develop their professional identities as CEs, identifies key influencing factors, and highlights effective strategies to foster CEs’ identity formation in informal and ad hoc SD settings.</p>