Using an 8-week mindfulness intervention, we investigated the mechanisms of mindfulness to address test anxiety in introductory physics II. Our goal was to explore the effectiveness of using an 8-week online mindfulness intervention to address student test anxiety. We used self-report measures to assess participants on mindfulness, emotional regulation, attentional regulation, and physics quiz scores at three-time points—time 1 (pre), time 2 (mid), and time 3. Participants in the mindfulness condition received 8 weeks of mindfulness training while the control group was used as a baseline for comparison with no mindfulness training. The 8-week online-based mindfulness intervention resulted in non-significant changes in test anxiety and quiz scores for the treatment compared to the control. Additionally, results from multiple regression modeling suggest that emotional regulation positively predicted mindfulness at time 3 (T3) of data collection, whereas attentional regulation negatively predicted mindfulness at T3. These findings have both theoretical and practical implications. Our findings suggest more research is needed to investigate the usefulness of mindfulness-based interventions in STEM-related settings.