Recent clinical trials demonstrate that fecal microbiota transplantation (FMT) enhances first-line immune checkpoint inhibitor efficacy in renal cell carcinoma, cutaneous melanoma, and non-small cell lung cancer with acceptable safety. Benefit appears mediated by functional microbiome remodeling, depletion of deleterious taxa, and systemic immunometabolic modulation, supporting microbiome-directed therapeutic strategies for cancer immunotherapy.