Research-Practice Partnerships (RPPs) are an emerging practice in Alberta, Canada, and internationally. In RPPs, researchers and teachers collaborate to address messy problems of practice in naturalized settings. Ideally, the enactment of partner research initiatives leads to a reciprocal and mutually beneficial learning process for researchers and practitioners to improve practice and inform theory. However, there can be challenges to creating effective research partnerships. While a number of RPPs utilize participatory research approaches to advance theory and practice, little is known about how they impact teacher professional learning and how they are experienced by teachers in schools. This case study of an RPP between the Werklund School of Education at the University of Calgary, the Galileo Educational Network Association, and a large urban school district in Western Canada examined: (a) how the RPP was initiated, enacted, and assessed; (b) how teachers learned as a result of participating; and (c) how professional knowledge developed by participant teachers was shared and supported at the school level. RPPs can be examined as transformative interventions for expansive learning in schools. By applying cultural-historical activity theory (CHAT) to the enactment of the RPP, this research developed an understanding of how participation in the RPP led to deep teacher professional learning and how teachers experienced improved agency impacting their professional identity within their school activity system as a result. By employing case study as a methodology, this research developed a thick description of teacher professional learning, agency, and identity within the RPP that was initiated to improve the assessment literacy of 26 junior high school teachers from five schools through sustained design-based professional learning. Findings illustrate that teacher professional learning in the RPP was characteristic of effective professional learning in the literature reviewed, and in addition, employed teacher leadership as a means of deepening teacher learning. Findings also show that teacher participants in this RPP played an important boundary-crossing role between the school and university activity systems requiring them to negotiate, translate, and contextualize their learning into the naturalized setting of their school, resulting in changes in their classroom practice, improved agency in their schools, and shifting professional identities. Study findings can be used to inform the design of teacher professional learning, instructional and teacher leadership practices, and the design and enactment of future RPPs.