STEM (science, technology, engineering, mathematics) education is widespread around the globe, with various theoretical frameworks and challenges from practical perspectives. In classroom practice, teacher readiness to conduct STEM learning is essential for its successful implementation. This study explores physics teachers’ readiness for STEM education using the Alignment, Capabilities, Engagement, or ACE, framework. Data collection is based on 101 teachers’ responses to six open-ended questions. Interestingly, all the teachers showed strong alignment with STEM education and how to implement it. Most of them have known STEM education as integrating technology, engineering, and mathematics to science (physics), but only about half of them have experience conducting STEM lessons. They have basic capabilities of identifying the possibilities of implementation in various physics curricula, such as motion, electricity, and fluids. However, in the online learning made necessary by the COVID-19 pandemic, the possibility of implementation is weakened. The teachers showed their engagement to explore more detail in designing and implementing STEM in their classrooms. Also reflected in the study was a significant challenge in terms of pedagogical and time management. Therefore, professional development in STEM education is essential to support teachers’ alignment, capabilities, and engagement to develop their readiness. As specific examples, STEM learning materials in motion, electricity, and fluids could help teachers understand the design and implementation of STEM education.