The purpose of this study was to determine the science teachers' perceptions of and beliefs about integrated science, technology, engineering, and mathematics (STEM) education in the science curriculum. A case study research method was used to determine teachers' perceptions and beliefs of integrated STEM education in the science curriculum. The research was conducted with the participation of ten science teachers who worked in secondary schools located in different regions of Turkey. In this study, semi-structured interviews were conducted for data collection. The data obtained from the study were analyzed using content analysis. During the content analysis process, open and axial coding steps that are based on grounded coding were followed. According to the findings, teachers' perceptions of STEM education knowledge were examined under three categories: STEM education concept knowledge, curriculum knowledge, and STEM education implementation knowledge. In the study, it was determined that most of the teachers considered themselves inadequately prepared for integrated STEM education in the science curriculum. When teachers' beliefs regarding the problems that might be encountered during an integrated STEM education process were examined, it was found that the emphasis was especially on teacher-based problems. Furthermore, teachers believed that integrated STEM education would have positive effects on teachers, students, and science education.