Anti-programmed cell death protein 1 (PD-1) or its ligand (PD-L1) are immune checkpoint inhibitors (ICIs) that have revolutionized cancer therapy. However, the efficacy of anti-PD-1 and anti-PD-L1 is limited by resistance and inter-individual variability. In recent years increasing evidence has highlighted the pivotal role of the gut microbiota in modulating the response to PD-1/PD-L1 immunotherapy. Extensive preclinical studies have demonstrated that commensal microbes can increase the efficacy of PD-1/PD-L1 blockade through multiple mechanisms, including the production of metabolites, such as short-chain fatty acids (SCFAs), tryptophan derivatives, and extracellular polysaccharides that remodel the tumor microenvironment, as well as the activation of immune pathways involving dendritic cells, CD8⁺ T cells, and M1 macrophages to increase antitumor immunity. Moreover, clinical studies have shown that fecal microbiota transplantation (FMT) and targeted probiotic interventions show promise for improving the response to PD-1/PD-L1 therapy, while reducing the risk of immune-related adverse events (irAEs). This review systematically explores the multifaceted regulatory roles of the commensal microbiota in PD-1/PD-L1 therapy and examines the preclinical prospects of microbiota-based personalized immunotherapeutic strategies. The integration of multiomics technologies, synthetic biology, and precise microbiota interventions may further optimize PD-1/PD-L1 immunotherapy and offer novel insights into antitumor immune modulation.