<p>Teaching assistants are increasingly important in assisting, facilitating, and supporting interdisciplinary undergraduate courses, especially among large research-intensive universities. Nonetheless, there is limited attention on their perceptions and experiences of process, content, and structure of their role identity formation and enactment. This study involves a series of individual interviews with 25 teaching assistants involving in the undergraduate interdisciplinary Common Core Curriculum at the University of Hong Kong. Using the framework of Dynamic Systems Model of Role Identity, this study illustrates these teaching assistants’ dynamic positionings as they construct and navigate their identities among the university, course instructors and teachers, and students. This study reveals that there is a variety of conceptualisations and constructions of identity among these teaching assistants in response to who they are and what it is that they do in the undergraduate interdisciplinary curriculum context. On the one hand, they remain passive and reactive when fitting themselves as teaching assistants with assigned roles and responsibilities in alignment with the institutional expectations and norms. On the other hand, they remain active and purposive under the drive of being and becoming teaching assistants they desire and aspire under their unique beliefs and actions. Such seemingly paradoxical tension reveals the conscious and agential identity construction among teaching assistants as mediated by and interacted with institutional structures and professional forces in the wider university landscape. This study contributes to the field by revealing how university teaching assistants involved in interdisciplinary education navigate the interplay between institutional cultures and frontline realities through individual and collective identity negotiation.</p>