This research was undertaken to understand the perceptions of the Physical Sciences teachers who participate in the South African ‘Eskom Expo for Young Scientists’, regarding the educational significance of the science fair, and the extent to which expo participation provides an opportunity for professional development. The educational significance of this article is found in its contribution to the professional identity of teachers in their roles as organisers, mentors and judges. The model of Beijaard et al. (Teach Teach Educ. 2004;20:107–128) was used to characterise the teachers’ professional identity in terms of professional knowledge, attitudes, beliefs, norms and values, as well as emotions and agency. Interviews with the Physical Sciences teachers were analysed using thematic analysis, ultimately interpreting and linking the categories of responses to the theme of professional identity. The study found that expo participation contributes to pedagogical knowledge, content knowledge (as both procedural and declarative or factual knowledge) and pedagogical content knowledge. Self-efficacy beliefs were strengthened, positive attitudes were developed, and strategies of inquiry-based learning and effective methodological instruction were observed during participation, which contributed to the participants’ school-based teaching. Teachers learn both from their engagement with learners, and through networking opportunities with fellow teachers. Teachers themselves value these aspects, and consequently, science fair participation is a sustainable form of professional development. It is recommended that the opportunity for professional development that is provided by teachers’ participation in such school-level science fairs should be acknowledged and promoted by schools and fair organisers.