Abstract Context Dietary fibers hold potential to influence depressive and anxiety outcomes by modulating the microbiota–gut–brain axis, which is increasingly recognized as an underlying factor in mental health maintenance. Objective Evidence for the effects of fibers on depressive and anxiety outcomes remains unclear. To this end, a systematic literature review and a meta-analysis were conducted that included observational studies and randomized controlled trials (RCTs). Data sources The PubMed, Embase, CENTRAL, CINAHL, and PsychINFO databases were searched for eligible studies. Data extraction Study screening and risk-of-bias assessment were conducted by 2 independent reviewers. Data analysis Meta-analyses via random effects models were performed to examine the (1) association between fiber intake and depressive and anxiety outcomes in observational studies, and (2) effect of fiber intervention on depressive and anxiety outcomes compared with placebo in RCTs. A total of 181 405 participants were included in 23 observational studies. In cross-sectional studies, an inverse association was observed between fiber intake and depressive (Cohen’s d effect size [d]: −0.11; 95% confidence interval [CI]: −0.16, −0.05) and anxiety (d = −0.25; 95%CI, −0.38, −0.12) outcomes. In longitudinal studies, there was an inverse association between fiber intake and depressive outcomes (d = −0.07; 95%CI, −0.11, −0.04). In total, 740 participants were included in 10 RCTs, all of whom used fiber supplements. Of note, only 1 RCT included individuals with a clinical diagnosis of depression. No difference was found between fiber supplementation and placebo for depressive (d = −0.47; 95%CI, −1.26, 0.31) or anxiety (d = −0.30; 95%CI, −0.67, 0.07) outcomes. Conclusion Although observational data suggest a potential benefit for higher fiber intake for depressive and anxiety outcomes, evidence from current RCTs does not support fiber supplementation for improving depressive or anxiety outcomes. More research, including RCTs in clinical populations and using a broad range of fibers, is needed. Systematic Review Registration PROSPERO registration no. CRD42021274898.