Objective: The use of mindfulness-based therapy (MBT) in oncology settings has become increasingly popular, and research in the field has rapidly expanded. The objective was by means of a systematic review and meta-analysis to evaluate the current evidence for the effect of MBT on symptoms of anxiety and depression in adult cancer patients and survivors. Method: Electronic databases were searched, and researchers were contacted for further relevant studies. Twenty-two independent studies with a total of 1,403 participants were included. Studies were coded for quality (range: 0-4), and overall effect size analyses were performed separately for nonrandomized studies (K = 13, n = 448) and randomized controlled trials (RCTs; K = 9, n = 955). Effect sizes were combined using the random-effects model. Results: In the aggregated sample of nonrandomized studies (average quality score: 0.5), MBT was associated with significantly reduced symptoms of anxiety and depression from pre- to posttreatment corresponding to moderate effect sizes (Hedges\'s g) of 0.60 and 0.42, respectively. The pooled controlled effect sizes (Hedges\'s g) of RCTs (average quality score: 2.9) were 0.37 for anxiety symptoms (p < .001) and 0.44 for symptoms of depression (p < .001). These effect sizes appeared robust. Furthermore, in RCTs, MBT significantly improved mindfulness skills (Hedges\'s g = 0.39). Conclusion: While the overall quality of existing clinical trials varies considerably, there appears to be some positive evidence from relatively high-quality RCTs to support the use of MBT for cancer patients and survivors with symptoms of anxiety and depression.