Mental health disorders, including depression, anxiety, bipolar disorder, and stress-related illnesses, represent a growing global health burden. Emerging evidence suggests that dietary interventions, particularly fermented foods, may influence mental health through modulation of the gut–brain axis. Fermented foods contain diverse microorganisms, bioactive metabolites, vitamins, peptides, and neurotransmitter precursors capable of influencing intestinal microbiota, immune regulation, neurotransmitter synthesis, and neural signaling. This review examines current evidence regarding the role of fermented foods in promoting mental well-being, emphasizing the biological mechanisms underlying gut–brain communication. Major pathways discussed include microbial modulation, vagal nerve signaling, immune regulation, endocrine interactions, neurotransmitter production, and microbial metabolite synthesis. Clinical and experimental evidence indicates that fermented foods such as yogurt, kefir, kimchi, sauerkraut, kombucha, tempeh, miso, and fermented dairy products may alleviate symptoms of depression, anxiety, and psychological stress. However, considerable heterogeneity exists among clinical studies regarding fermented food composition, dosage, microbial strains, and study duration. While fermented foods demonstrate considerable promise as complementary therapeutic agents in nutritional psychiatry, additional large-scale randomized controlled trials are required to establish standardized dietary recommendations.