The challenge of the industrial revolution 4.0 (4IR) demands that future generations have high knowledge, skills, and capabilities in technology. A country’s competitiveness depends on STEM human capital as 4IR develops. Therefore, more organized and holistic actions need to be taken to ensure that the STEM level of mastery is enhanced among students. High demand for STEM human capital is a challenge to most developed countries in the world. Teachers are a crucial medium in the implementation of STEM education in schools. Mastering the knowledge of teachers in STEM education will provide positive input to students’ interest in learning. The purpose of this study was to identify the mathematics teacher’s knowledge of STEM-based education. A quantitative study was conducted on 66 teachers - who teach STEM subjects in high school. Twenty-three of them are math teachers. The findings show that these teachers know STEM education at a moderate level. The implications of this study indicate that the mastery of teachers ‘knowledge of STEM education will stimulate students’ interest in STEM education. Future studies can explore, the factors that influence teachers’ knowledge of STEM education in terms of teacher preparation and in-service teacher professional development programs.