The purpose of the present study was to investigate the impact of an intervention on primary school students' construction of knowledge on ocean acidification and the development of their systems thinking. Eighty-five 11 to 12-year-old students from five different classes of two public primary schools in Greece participated in the 8-h intervention. The intervention included inquiry-based and knowledge-integration activities, and students worked in groups during all activities. Rich pictures, made by the groups at the beginning and the end of the intervention, were used to evaluate their progress in their knowledge concerning the carbon cycle, as well as in their systems thinking. Our findings showed that the intervention contributed to primary students' conceptual knowledge of the carbon cycle and the inclusion of ocean acidification in the carbon cycle. It also helped them improve their systems thinking, indicating that students' systems thinking at this age could be developed through formal instruction with interventions which emphasize content knowledge and use an earth systems approach. Moreover, our findings indicate that the systems thinking perspective can serve as an effective approach to help children better understand and critically engage with complex environmental issues, such as ocean acidification.