This study determines the critical success factors of teaching mathematics to special education (SPED) students wherein a list of success factors is identified through a literature survey and analyzes the causal relationships among the identified factors to evaluate the key success factors using the integration of the grey system theory and decision-making trial and evaluation laboratory (DEMATEL), named as the grey-DEMATEL. Results reveal that professional development (PD), institutional support (IS), and individual factors (IF) are categorized as net causes. They influence other factors considered as net effects, including mentoring, self-efficacy, teaching capacity, student feedback, teaching knowledge, instructional accommodation, and use of technology. Among those net causes, only PD yields as the key factor, while IS and IF are the minor key factors. Thus, decision-makers must allocate resources that provide PD to teachers in delivering SPED mathematics teaching. A sensitivity analysis suggests these findings are robust to linguistic evaluation scale changes. The insights outlined in this study would aid educational managers and decision-makers of educational institutions in carefully designing initiatives to improve the quality of mathematics education provided to SPED students. Some potential directions for future research agenda are also discussed.