Mindfulness-based interventions (MBIs) have emerged as alternative treatments to treat anxiety and stress-related disorders. The aim of this study is to investigate the effectiveness of MBIs if compared to control interventions and cognitive behavioral therapy (CBT) to improve internalizing symptoms in patients with anxiety and stress-related disorders. We performed a systematic review and multilevel meta-analysis of randomized controlled trials. Outcomes were anxiety, stress, and depression rating scales (internalizing symptoms), further classified as pertaining to distress and fear symptom domains. A total of 40 outcomes were retrieved from 10 studies. MBIs were superior to control interventions for internalizing and distress, but not for fear symptoms. CBT was not superior to MBIs for internalizing and distress, but CBT was superior to MBIs for fear symptoms. The majority of the included studies have a high risk of bias and small sample size, and we found evidence of publication bias for some outcomes. Therefore, results should be interpreted cautiously. MBIs seem promising therapeutic options for treating distress symptoms in anxiety disorders; however, more research is needed to recommend MBIs as first-line treatments. MBIs do not seem effective to treat fear symptoms, for which CBT is significantly more effective.