This study examines the role of critical professional learning in leading socially just schools. The study investigated the experiences of teachers who are the members of Egitim-Sen, an active teacher union in Turkey. The article first focuses on the relationship between professional development and teacher unions followed by the definition of the roles of teachers as social justice leaders in constructing socially just schools. Then, it presents findings from a qualitative study of 12 Egitim-Sen member teachers with different experiences. The data were analysed using the phenomenological research design and presented under the themes of 'critical professional learning experiences', 'injustice in schools', and 'union member teachers as social justice leaders'. The findings indicate that the union member teachers take part in union training in various forms and that these experiences contribute to raising teachers' awareness of injustices in their schools with their critical professional learning experiences. Union member teachers play active roles in constructing socially just schools both individually and as organised union members.