National reform documents all around the world issue an urgent need for the development of integrated STEM (Science, Technology, Engineering, and Mathematics) curricula. With the emphasis on the development of STEM curricula, there is also a need for improving teachers' and future teachers' conceptions of the integrated STEM approach. This study responds to this call by providing a context to develop pre-service science teachers' understanding of STEM disciplines. Query: In this study, we explored how the engineering design-based intervention developed the pre-service science teachers' understanding of integrated STEM education. For this purpose, four STEM activities that focus on a daily-life problem and an iterative engineering design process to solve the problem were adopted. The results revealed that participants' understanding of science was developed throughout the activities (i.e., from the first to the last activity). Similarly, there was an enrichment in their understanding of the engineering design process. On the other hand, there was no gradual development in participants' understanding of mathematics implying that participants had difficulties in making explicit connections with mathematics throughout the activities. Implications for effective STEM courses for developing pre-service science teachers' understanding of STEM disciplines and STEM courses in pre-service science teacher education programs were discussed.