<p>The present study aimed to investigate the efficacy of mindfulness-based stress reduction (MBSR) in alleviating rumination and anxiety sensitivity among high school students with social anxiety disorder. This quasi-experimental study employed a pre-test-post-test design with a control group. The target population consisted of all high school students in Ahvaz, Iran, exhibiting social anxiety disorder symptoms during the 2023 academic year. Students who scored above the average on the Social Phobia Inventory and received a clinical diagnosis of social anxiety disorder from a clinical psychologist were included in the study. A total of 46 participants were randomly assigned to either the treatment (<i>n</i> = 23) or no treatment (<i>n</i> = 23) group. Participants were assessed using the Social Phobia Inventory, the Ruminative Response Scale, and the Anxiety Sensitivity Index. Data were analyzed using repeated-measures ANOVA. At pre-test, no significant group differences were observed for either rumination or anxiety sensitivity. However, post-test scores revealed significant reductions in both rumination and anxiety sensitivity within the treatment group compared to the no-treatment group (<i>p</i> &lt; 0.01). These findings suggest that MBSR holds promise as an intervention for mitigating rumination and anxiety sensitivity in adolescents with social anxiety disorder. Future research should investigate the long-term efficacy of MBSR and explore its potential mechanisms of action.</p>