Learning about sustainability issues is crucial for upper secondary students as they will enter adulthood within a short time. This study aimed to explore how science teacher teams in Swedish upper secondary schools can develop their knowledge and expertise for teaching about sustainable development (SD) through collective reflections with the support of the reflective tool Content Representation (CoRe). Twelve in-service science teachers participated in a study to investigate their development of Pedagogical Content Knowledge (PCK) for teaching SD. The qualitative research design included semi-structured interviews and audio-recordings from collective meetings with science teacher teams. The findings covered four themes about how the collective reflections with support of the CoRe tool stimulated teachers’ collective PCK development for teaching SD: (1) Creating structure and a focus for learning conversations, (2) Modification of the teaching approach, (3) Development of new content knowledge and pedagogical knowledge in SD, and (4) Shared language to stimulate equal opportunities for students to learn SD. As such, the results from the study provides important implications for how reflection among science teacher teams can be stimulated to promote teachers’ knowledge development for teaching about SD.