This study aims to implement a science, technology, engineering, and mathematics (STEM) education approach in an environmental education course. The research involved the design and implementation of STEM activities by researchers, as part of the environmental education course taught in the second year of a Primary School Teaching undergraduate program. In this study—a case study using qualitative research methods—after the implementation of STEM activities, some mind maps and the views of the pre-service teachers regarding STEM education were examined. A total of 42 pre-service teachers participated in the study and an examination of their mind maps showed that they have a rich conceptual structure regarding STEM education and also associate STEM fields both with one another and with environmental education. In the interviews conducted following the activities, the pre-service teachers stated that they found STEM education to be efficient, easy to retain, and fun.