Purpose of Review <p>To provide an updated overview of the role of the human microbiome in the initiation, progression, and therapeutic response of gastrointestinal tumors, emphasizing molecular, immunological, and metabolic mechanisms, as well as its potential as a target for novel therapeutic strategies.</p> Recent Findings <p>Emerging evidence demonstrates that microbiome dysbiosis contributes to carcinogenesis across gastrointestinal malignancies, including colorectal, gastric, hepatic, and pancreatic cancers. Microbial-derived metabolites, such as short-chain fatty acids and secondary bile acids, modulate key signaling pathways involved in cell proliferation, apoptosis, and genomic stability. In addition, the microbiome influences the tumor microenvironment and immune responses, shaping variability in treatment outcomes. Both preclinical and clinical studies have shown that microbiome composition affects the efficacy and toxicity of chemotherapy and immunotherapy. Notably, specific microbial signatures are being explored as non-invasive biomarkers for early detection and prognostic stratification, while microbiome modulation strategies, such as diet, probiotics, antibiotics, and fecal microbiota transplantation, have demonstrated potential to enhance therapeutic response.</p> Summary <p>The bidirectional interaction between the microbiome and the host plays a central role in gastrointestinal tumorigenesis and treatment response. Although this field holds significant promise for precision oncology, its clinical translation remains limited by interindividual variability, methodological heterogeneity, and insufficient longitudinal evidence. Future efforts should focus on standardization, validation of microbiome-based biomarkers, and integration of multi-omics and artificial intelligence approaches to enable clinically actionable applications.</p>