Purpose: The role of basic science teachers (BSTs) in medical education has been changing dynamically. Less is known, however, about how BSTs perceive their professional identity and what factors influence its formation. This study aims to explore how the professional identity of BSTs is formed and what factors influence this professional identity formation (PIF) using the 4S (“Situation, Self, Support, Strategies”) Schlossberg framework. Method: A qualitative descriptive study using focus groups (FGs) was conducted. Maximum variation sampling was used to purposively select BSTs. A rigorous thematic analysis was completed, including independent thematic analysis, intermittent checking and iterative discussions among researchers, and member checking. Results: Nine FGs, involving 60 teachers, were conducted. The findings highlighted four major themes reflecting the 4S framework: the self as internal driver, early-career events and opportunities, individual and institutional support, and active participation in continuing professional development. Both the “Self” and the “Situation” components prompted the BSTs to utilize supports and enact strategies to become professional teachers. Although the BSTs in this study were primarily internally driven, they relied more on existing support systems rather than engaging in various strategies to support their growth. Conclusion: It is important to address the PIF of BSTs given their dynamic roles. Looking through the lens of the 4S framework, PIF is indeed a transition process. A structured, stepwise faculty development program, including mentorship, reflective practice, and a community of practice designed to foster BSTs’ identities, should be created, taking into consideration the diverse factors influencing the PIF of BSTs. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.