Graduate Teaching Assistants (GTAs) are doctoral researchers who are paid to teach. Building on work exploring identity construction in doctoral education and in the GTA context, this study uses role identity theory (McCall and Simmons 1978) as a theoretical framework to explore the factors influencing GTAs' teacher identity development. Role identity theory emphasises the importance of social interaction and negotiation of identity with those in similar and opposing roles. Semi-structured interviews were conducted with nine GTAs from a variety of STEMM disciplines. From a role identity perspective, we found that the development of GTA teacher identities was strongly influenced by the behaviours and values of others, whilst maintaining the autonomy of the individual. We conclude with a set of recommendations for academic leadership and practice that consider the institutional cultures within which teaching and learning are situated to suggest how GTAs can be supported to develop their identities as teachers.