We present teacher-researcher partnership (TRP) as a way of fostering teachers' professional learning. Teachers' participation as research group members is an essential aspect of the partnership. Teachers and researchers share the same goal, which is to improve their understanding of and enhance students' engagement in science. Project-based learning (PBL) was selected as a means of enhancing student engagement. The activities of the partnership focused on the co-design and enactment of and co-reflection on PBL units. Teachers participated in the design of the data collection process and the interpretation of initial findings. As an indicator of teachers' professional learning, we examined students' engagement during different implementations of the PBL units. Student engagement was measured using a situational experience sampling questionnaire delivered via mobile phones. The students' experiences of scientific practices and engagement in actual learning situations were measured in the first and second years of the teachers' implementation of the teaching units. An analysis of the students' responses showed that the students were 20% more engaged in the second year than in the first year. We argue that TRP has the potential to enhance teachers' professional learning.