This investigation was performed to determine the impacts of the STEM approach in environmental education (EE) on environmental attitudes and knowledge levels and to examine student opinions on the STEM approach in EE. The investigation was implemented in a government school in Turkey in 12 weeks using a mixed method. A Quasi-experimental design with a pretest-posttest control group and a phenomenological research design were used in the quantitative-qualitative parts of the investigation, seriatim. Investigation data were collected using the Environmental Attitude Scale, Environmental Knowledge Test, and an Interview Form, which were also administered as follow-up tests to evaluate the permanence of the changes in attitudes and knowledge. The experimental group received EE with the STEM approach while the group under control received the 5E model. In both study groups, EE was based on reusing waste materials. The study results showed that the 5E model and STEM approach in EE had a meaningful effect on the rise in students' attitudes and knowledge levels toward the environment. However, the STEM approach in EE was more effective compared to the 5E model. In addition, pupils in the experimental group had positive opinions regarding the STEM approach used in EE. Considering the research results, it was proposed that the STEM approach be included in EE.