BACKGROUND: Gut microbiota play a critical role in mediating the bidirectional association between cancer and depression. Emerging evidence indicates that adjusting the dietary component intake can significantly alter gut microbiota composition, thereby influencing the host's metabolism and immune function. Changes in gut microbiota and their metabolites may represent key factors in preventing cancer-depression comorbidity. METHODS: English publications were searched in databases including the Web of Science, Scopus, and PubMed using a series of keywords: "cancer", "depression", "gut microbiota", "dietary components", and related terms, individually or in combination. The search focused on preclinical and clinical studies describing the regulatory effects of dietary component interventions. RESULTS: This narrative review summarizes the associations among gut microbiota, cancer, and depression, and synthesizes current evidence on the modulatory effects and mechanisms of specific dietary component interventions, including dietary patterns, probiotics, prebiotics, and diet-derived phytochemicals, on gut microbiota. On the one hand, these interventions inhibit abnormal proliferation signals in the tumor microenvironment and enhance anticancer immune responses; on the other hand, they modulate neurotransmitter homeostasis, suppress neuroinflammation, and improve mood behaviors through the gut-brain axis interactions mediated by microbial metabolites. CONCLUSIONS: The complex associations among cancer, depression, and gut microbiota require further clarification. Modulating gut microbiota composition through dietary components represents a novel therapeutic strategy for improving cancer-depression comorbidity. Regulated gut microbiota enhance immune homeostasis and intestinal barrier function, while their metabolites bidirectionally modulate one another via systemic circulation and the gut-brain axis, thereby improving both the tumor microenvironment and depressive-like behaviors in cancer patients while reducing the adverse effects of cancer.