<p>Cancer has become one of the most prominent causes of death worldwide due to its increasing mortality rate. Immune checkpoint blockade therapy is an effective regimen for tumor control. Still, it faces challenges, including primary resistance and interindividual variations, thereby directing the field towards a new era of immunotherapy adjuncts. Recent studies have shown that the microbiota of cancer patients influences the outcomes of ICB (immune checkpoint blockade) therapy through microbiome-immune system crosstalk. Homeostatic commensal microbial consortia aid in combating tumors by enhancing immunity, whereas dysbiotic microbes facilitate cancer development by mediating immunosuppression. Microbiota modulation via microbiome-targeted interventions, including fecal microbiota transplantation or washed microbiota transplantation from responders, biotic supplements, and dietary modifications, can convert primary resistance to durable responses and thus augment immunotherapy responsiveness in cancer treatment. This review discusses the dual nature of microbiota in mediating the development and treatment of cancer, its crucial role in shaping ICB therapy responsiveness, and the identification of microbial biomarkers into a refined Discovery-Validation-Clinical (DVC) pipeline linked to multi-omics profiling and personalized microbiome-modulation interventions for ICB therapy optimization. In addition, it presents the translational clinical decision framework that highlights patient stratification, intervention timing, and implementation barriers to support clinical translation. Ultimately, the gut microbiome emerges as a “force multiplier” of cancer ICB therapy, thereby enabling microbiome-guided precision oncology and strengthening a roadmap toward personalized cancer care.</p>