The current article provides an overview of a Teacher Professional Development (TPD) program that has been designed, pilot tested, and implemented to investigate the impact of augmented reality (AR) on: (a) Teachers' level of technology (AR) acceptance, adoption of inquiry-based instructional approaches, and confidence towards teaching twentieth-first century skills in STEM-related courses; and (b) students' potential enhancement of specific twentieth-first century skills and motivation and interest during a STEM- (science, technology, engineering, mathematics)-related course supported with AR. This article focuses on the teachers' points of view concerning the impact of their STEM-related interventions on their students' motivation and learning, as well as the factors that influence the teachers' technology acceptance. The TPD program has been implemented in Cyprus and Greece with twenty-five lower secondary school teachers (20 in Cyprus and 5 in Greece). The research methodology applied is Educational Design Research (EDR), including an initial phase of the TPD program and a second (improved) phase. The data collection tools consisted of questionnaires, interviews, and observation of classroom interventions. Initial findings and their implications for teaching and future research are discussed, indicating the potential benefits and challenges surrounding the integration of AR within the educational process.