Abstract: Gut microbiota (GM) are composed of trillions of bacteria, fungi, archaea, parasites, and viruses. The composition of GM varies among individuals; is related to habits, race, genetics, age, and previous drug use; and participates in physiological functions such as metabolism, inflammation, immunity, and tumor suppression. The potential role of GM as a tumor biomarker is clinically significant for the safety, tolerability, and efficacy of supportive cancer therapy. The relationship between GM and cancer is complex and close. GM can be used as a biomarker, diagnostic tool, or therapeutic target. GM are indispensable in the direct treatment of cancer and can regulate, diagnose, and prognosticate gastrointestinal tumors, including colorectal cancer and gastric cancer, and may also act as a biomarker for malignant tumors of other organ systems such as lung cancer. Improving the diversity of GM, reducing antibiotic use, and decreasing the abundance of harmful GM may effectively improve the prognosis of tumors and the efficacy of antitumor therapies. The study aimed to summarize the role of GM in the prevalence and development of malignant tumors; the efficacy and side effects of radiotherapy, chemotherapy, and immunotherapy; and the effect of diet (and subsequently GM) on tumors in order to lay the foundation for future research into the accurate regulation and detection of GM in both the treatment and prognostication of tumors.