The gut microbiota has emerged as a critical determinant of antitumor immunity and a potential modulator of responses to immune checkpoint inhibitors (ICIs). Although pre-clinical and clinical studies suggest that specific bacterial taxa may influence both efficacy and immune-related adverse events (irAEs). However, the magnitude and consistency of these associations remain unclear. A systematic search of PubMed, Embase, Web of Science, and the Cochrane Library was conducted through March 2025. Eligible studies evaluated baseline gut microbiota composition, fecal microbiota transplantation (FMT), probiotic/prebiotic interventions, or antibiotic exposure in cancer patients treated with ICIs. Pooled hazard ratios (HRs) for overall survival (OS) and progression-free survival (PFS), and odds ratios (ORs) for response rates and irAEs, were estimated using random-effects models. Across 38 studies involving 5,642 patients were included. Pooled analysis demonstrated that enrichment of Akkermansia muciniphila, Bifidobacterium longum and Faecalibacterium prausnitzii was significantly associated with improved OS (HR 0.62, 95% CI 0.51-0.76) and PFS (HR 0.69, 95% CI 0.55-0.83). Conversely, antibiotic exposure before or during ICI treatment was associated with worse OS (HR 1.84, 95% CI 1.45-2.34). Patients undergoing FMT from responders exhibited higher objective response rates (OR 2.91, 95% CI 1.48-5.73). Microbiota diversity indices were consistently higher in responders than in non-responders. Collectively, gut microbiota composition and its modulation significantly impact the therapeutic efficacy and toxicity profile of ICIs. These findings highlight the translational potential of microbiome-based biomarkers and interventions in optimizing immunotherapy.  Kulcsszavak: gut microbiota, immune checkpoint inhibitors, cancer, fecal microbiota transplantation, probiotics, antibiotics