This literature review explores the impact of increased dietary fiber intake, compared with a low-fiber diet, on gut microbiota composition in adults with symptoms of mood disorders. This uses seven central studies, including randomized controlled trials, a long-term cohort study, and a systematic review. These studies showed that higher fiber intake is associated with improved mood symptoms and increased microbial diversity, particularly with whole-diet interventions rather than supplementation alone. There is a broad correlation between fiber intake and reduced symptoms of depression and anxiety in observational studies, while dual-outcome trials measuring both mental health and gut microbiome outcomes demonstrate that fiber modulates gut behavior and supports the gut-brain axis. Although many sources carried propitious results, most trials had short durations, small sample sizes, and lacked standardized fiber interventions, limiting the ability to establish causality. A critical gap in the literature exists because only one study directly measures both gut microbiome composition and psychological outcomes. Despite these limitations, this review suggests that dietary fiber intake has high potential as an accessible strategy to alleviate symptoms of mood disorders by enhancing gut microbiota composition. Future research should prioritize long-term, large-scale trials with standardized interventions and dual-outcome endpoints to establish fiber as a non-pharmacological treatment for mood disorders.