Phenomenon: This study explores professional identity formation during a final year of medical school designed to ease the transition from student to practitioner. Although still part of the undergraduate curriculum, this “transitional year” gives trainees more clinical responsibilities than in earlier rotations. Trainees are no longer regarded as regular clerks but work in a unique position as “semi-physicians,” performing similar tasks as a junior resident during extended rotations. Approach: We analyzed transcripts from interviews with 21 transitional-year medical trainees at University Medical Center Utrecht about workplace experiences that affect the development of professional identity. We used Social Identity Approach as a lens for analysis. This is a theoretical approach from social psychology that explores how group memberships constitute an important component of individual self-concepts in a process called ‘social identification.’ The transcripts were analyzed using thematic analysis, with a focus on how three dimensions of social identification with the professional group emerge in the context of a transitional year: cognitive centrality (the prominence of the group for self-definition), in-group affect (positivity of feelings associated with group membership) and in-group ties (perception of fit and ties with group members). Findings: Students were very aware of being a practitioner versus a student in the position of semi-physician and performing tasks successfully (i.e., cognitive centrality). Students experienced more continuity in patient care in transitional-year rotations than in previous clerkships and felt increased clinical responsibility. As a semi-physician they felt they could make a significant contribution to patient care. Students experienced a sense of pride and purpose when being more central to their patients’ care (i.e., in-group affect). Finally, in extended rotations, the trainees became integrated into daily social routines with colleagues, and they had close contact with their supervisors who could confirm their fit with the group, giving them a sense of belonging (i.e., in-group ties). Insights: Using the three-dimension model of social identification revealed how students come to identify with the social group of practitioners in the context of a transitional year with extended rotations, increased clinical responsibilities, and being in the position of a “semi-physician.” These findings shed light on the identity transition from student to practitioner within such a curricular structure.