To transition successfully into independent practice, newly graduated independent physicians (new "attendings") undergo a process of professional identity formation (PIF) as a clinician within a new community of practice (CoP). PIF is crafted by socialization within a CoP including transfer of tacit knowledge. While certain tacit knowledge is critical for professional identity, we understand little how it shapes PIF. We set out to describe the tacit knowledge acquired by new attendings within a CoP and how it contributes to PIF. Informed by constructivist grounded theory, we interviewed 23 new attendings about the tacit knowledge they acquired in early practice. Data collection and analysis occurred iteratively. We identified themes using constant comparative analysis and generated a theory that underwent member checking and feedback. Implicit standards from group culture imparted high expectations on new attendings and led to internal stress. New attendings also encountered a tacit code of conduct as behavioral elements of group culture. These elements created external conflict between new attendings and group members such as departmental colleagues, consulting physicians, and other health professionals. Depending on the support they received, new attendings responded to the stress and conflict in three ways: they doubted, adjusted, or avoided. These strategies molded their professional identity, and moved them towards or away from the CoP as they navigated their transition and PIF. We describe a novel theory of how tacit group culture shaped new attending physicians' professional identity in a new community of practice. Internal stress and external conflict occurred due to high expectations and tacit culture elements. New attendings' doubt, adjust, or avoid responses, shaped by support they received, in turn crafted their professional identity. Education leaders should prepare graduating trainees to navigate aspects of transition to independent practice successfully.