Purpose <p>This study evaluated the efficacy of an online mindfulness-based stress reduction (MBSR) intervention in reducing anxiety among breast cancer (BC) survivors, leveraging the potential advantages of digital delivery to enhance psychosocial care for this population.</p> Methods <p>In this randomized controlled trial, 68 BC survivors with elevated anxiety levels were assigned to either an 8-week online MBSR intervention (<i>n</i> = 33) or a waitlist control group (<i>n</i> = 35). Anxiety (GAD-7), depression (PHQ-9), fatigue (FACIT-F), insomnia (ISI), cancer-related worry (CWS), vasomotor symptoms (MENQOL), and mindful awareness (MAAS) were assessed at baseline, post-intervention, and at 5- and 8-months follow-up.</p> Results <p>Compared to the control group, MBSR participants showed significant reductions in anxiety (mean difference -4.13 points, 95CI -6.79 to -1.46, <i>p</i> = 0.003), depression (mean score difference -6.03, <i>p</i> &lt; 0.001), fatigue (mean difference + 6.03, <i>p</i> = 0.002), insomnia (mean difference -3.97, <i>p</i> = 0.026), and cancer-related worry (mean difference -4.57, <i>p</i> = 0.003) at post-intervention, but no change in vasomotor symptoms (<i>p</i> &gt; 0.05). MBSR participants also demonstrated increased mindful awareness (mean difference + 1.00, <i>p</i> = 0.004) that persisted through follow-up. The proportion of participants with clinically significant anxiety decreased from 96% pre-intervention to 38% at 8-months follow-up in the MBSR group, compared to&#xa0;relatively stable rates (88% to 87%) in the control group.</p> Conclusions <p>Online MBSR is a potentially effective intervention for reducing anxiety and improving psychological well-being in BC survivors. This accessible format may help overcome barriers to psychosocial care for cancer survivors.</p> ClinicalTrials.gov ID <p>NCT05837169.</p>