Immunotherapies, including immune checkpoint inhibitors and chimeric antigen receptor-T cell therapies, depend heavily on a healthy and diverse gut microbiome for optimal efficacy. Dysbiosis, or an imbalance in gut microbial composition and function, can diminish immunotherapy responses by altering immune cell trafficking and metabolic output. Key microbial metabolites such as short-chain fatty acids and modified bile acids shape host immunity and influence T-cell function, while their disruption can foster an immunosuppressive microenvironment. Emerging strategies to restore a balanced microbiome and boost treatment outcomes include dietary interventions, supplementation with beneficial microbes, and fecal microbiota transplantation. Despite these advances, challenges remain in defining dysbiosis, identifying reliable biomarkers, and tailoring microbiota-centered interventions. Nevertheless, as our understanding evolves, the gut microbiome holds promise as an integral component of personalized cancer immunotherapy.