Gut microbiota can modulate antitumor immunity and influence immune checkpoint blockade (ICB) therapy efficacy and treatment-associated toxicity. Variations in the therapeutic effect of ICB among individuals are partially attributed to microbiota. This review summarizes current knowledge on how specific bacterial species enhance or hinder ICB outcomes by regulating immune cell activation, antigen presentation, and systemic inflammation. The review further outlines translational strategies to optimize ICB, including microbiota-targeted interventions (e.g., prebiotics, fecal microbiota transplantation, and metabolite therapies) to overcome resistance and mitigate treatment-related toxicities, focusing on immune-related colitis. Additionally, emerging microbial biomarkers in melanoma, lung cancer, and hepatobiliary cancers that predict ICB response are discussed, highlighting the gut microbiome as a potential target for personalized cancer immunotherapy. By integrating mechanistic insights with clinical evidence, this review underscores the potential of microbiota-centered approaches to improve patient outcomes in ICB-based treatments, emphasizing the pivotal role of the gut microbiota in modulating both therapeutic efficacy and immune-related adverse events.Copyright © 2025 The Chinese Medical Association, produced by Wolters Kluwer, Inc. under the CC-BY-NC-ND license.