It is vital that upper secondary students gain a rich and deep knowledge of sustainability issues, as they will enter adulthood and working life within a short time. The students belong to a generation that will be intensely involved in managing several environmental issues to achieve sustainable development for our Earth. However, earlier research indicates that many teachers have a low self-efficacy for teaching sustainability issues. This study aimed to explore how science teacher teams in Swedish upper secondary schools can develop their knowledge and expertise in sustainable development (SD) through collective reflections with the support of the reflective tool Content Representation (CoRe). Science teachers' pedagogical content knowledge (PCK) development in teaching SD was examined. Twelve in-service science teachers participated in the study. The qualitative research design included semi-structured interviews and science teacher teams' collective meetings. The findings covered four themes about how the collective reflections with support of the CoRe tool stimulated teachers' PCK development: (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. The conclusion is that with the support of CoRe, the teacher team's collective PCK in SD was developed; therefore, we recommend that decision-makers within the school system organize regular meetings in science teacher teams and introduce CoRe to the teachers.