Research-practice partnerships (RPPs) have recently emerged as powerful tools for bridging theory-practice gaps in educational research. These long-term collaborative initiatives between researchers and practitioners aim to advance schools and educational systems through research, fostering opportunities for educational researchers and practitioners to contribute diverse forms of knowledge. This study examines knowledge dynamics in an RPP through the lens of relational agency in three distinct but interconnected action research projects led by three teachers and four researchers. The study is theoretically grounded in cultural-historical activity theory, drawing on the conceptual framework of relational agency and using empirical data comprising participants’ written reflections, observations of group meetings, and interviews with participants. It was found that the participants’ knowledge expanded mainly in three domains: research, teaching, and collaboration. This facilitated the development of relational expertise, common knowledge, and relational agency, which in turn helped expand the object of collaboration. The dialectical interplay between affinity (i.e. what united teachers and researchers) and alterity (what distinguished them) was vital in the expansion of knowledge.