The research study aimed to determine the combined effect of math anxiety and student engagement on math proficiency of STEM students, which served as basis for teaching intervention plan. Using descriptive-correlational quantitative design, the study was conducted among one hundred sixty-seven STEM learners selected via stratified random sampling. Adopted survey questionnaires were administered to determine the level of math anxiety and student engagement, its individual relationship with, as well as its combined effect, on math proficiency. The findings revealed that math anxiety negatively affects math proficiency, while student engagement was on the positive side. Thus, math anxiety and student engagement, when considered together, posed a significant combined effect on math proficiency which shows that as math anxiety increases, math proficiency declines, whereas high levels of engagement lead to greater development in math proficiency. The results of this study were used as the basis to propose a teaching intervention plan aimed at alleviating math anxiety, sustaining and improving engagement, and enhances math proficiency.