In this study, we aimed to identify how the learning activities elicited in a lesson study project contributed to self-perceived change in supervisors’ pedagogical content knowledge (PCK). Lesson study is a method which combines both professional and educational development. During a lesson study project, teachers collaborate in a team and develop, teach, evaluate, and redesign a research lesson. During the 4-month lesson study project described here, four supervisors designed a protocol for research supervision meetings aimed at enhancing undergraduate students’ learning. During the project, they experimented with open questioning and giving positive feedback instead of giving instruction and explanations. A mixed-methods design was used in this study. Data on the supervisors’ learning activities and PCK were gathered using learner reports, video-recordings of meetings, and exit interviews. The analyses of these data showed that the lesson study project contributed to the development of the supervisors’ PCK on instructional strategies and student understanding. The learning activity that contributed most to these changes was reflecting on their own practice and that of their students.