We explore the professional identities of UK-based secondary science teachers who actively participated in science research for at least six months. The study uses thematic analysis to analyse semi-structured interviews with 17 participants across England and Scotland, from a variety of educational/socio-economic contexts. We found that through participation in research projects, teachers develop a multi-faceted sense of professional identity that includes the roles of teacher, scientist/researcher, mentor and coach. Teachers who are research-active develop complex professional networks that have a positive impact upon their sense of professional worth and self-belief. Through participation in research, teachers identified as both science teachers and scientists and this has been encapsulated in this research as a transition in professional identity to 'teacher scientist'. The key enabling factor in identification as a 'teacher scientist' is a teacher's positive interaction with scientists/researchers. Teachers are motivated to participate in research projects in response to the enthusiasm of their students and a desire for students to contribute to research that could provide solutions to real-world challenges. This understanding of the capacity of science teachers to become 'teacher scientists', and recognising teachers' altruistic motivations, could contribute to teacher retention and recruitment strategies that are less focused on financial incentives.