The gut microbiota plays a significant role in the development and progression of gastrointestinal tumors, influencing tumor growth through dysbiosis, bacterial toxin release, and modulation of host signaling pathways and immune status. Immune checkpoint inhibitors (ICIs) have significantly improved cancer treatment efficacy by enhancing immune cell responses. Current clinical and preclinical studies have shown that the gut microbiota and its metabolites can enhance the effectiveness of immunotherapy, and certain gut microbiota can serve as biomarkers for predicting immunotherapy responses. Interventions targeting the gut microbiota, such as fecal microbiota transplantation, probiotics, prebiotics, engineered bacteria, and dietary interventions, have shown promise in improving the efficacy of ICIs and enhancing immunotherapy outcomes. The gut microbiota consists of bacteria, fungi, viruses, archaea, and potentially protozoa, and it interacts with host genes to modulate growth processes and adapt to environmental exposures. It plays a vital role in maintaining systemic immune homeostasis during bacterial infections and is integral to various immune processes. The intestinal mucosal barrier, composed of tight junction proteins, serves as a defense mechanism to separate bacteria from host cells. Alterations in the epithelial membrane can increase susceptibility to infections and facilitate the transfer of microbial metabolites to the host. In colorectal cancer (CRC), patterns of imbalance or disturbance in the gut microbiota have been recognized as indicators of specific diseases or poor health. Bacterial dysbiosis, impaired intestinal mucosal barrier, and cascade signaling pathways are key factors in CRC development. Bacterial toxins, such as those produced by *Clostridium perfringens* and *Fusobacterium nucleatum*, play a pivotal role in the development and progression of CRC. Inflammatory responses induced by the gut microbiota can contribute to the initiation and progression of CRC. The gut microbiota also affects other digestive tumors, including gastric cancer, hepatocellular carcinoma (HCC), and pancreatic ductal adenocarcinoma (PDAC). Short-chain fatty acids (SCFAs) and bile acids are important metabolites produced by the gut microbiota that contribute to improved gut health and regulate host metabolism. The gut microbiota and its metabolites have complex and diverse effects on the immune system, influencing both innate and adaptive immunity. In the tumor microenvironment, the gut microbiota impacts tumor progression through its metabolites, genotoxins, and signaling pathways. Gut microbiota composition can influence the response to ICIs, and specific gut microbiota can enhance the efficacy of immunotherapy. Interventions targeting the gut microbiota, such as fecal microbiota transplantation, probiotics, and prebiotics, have shown promise in improving the efficacy of ICIs and enhancing immunotherapy outcomes.The gut microbiota plays a significant role in the development and progression of gastrointestinal tumors, influencing tumor growth through dysbiosis, bacterial toxin release, and modulation of host signaling pathways and immune status. Immune checkpoint inhibitors (ICIs) have significantly improved cancer treatment efficacy by enhancing immune cell responses. Current clinical and preclinical studies have shown that the gut microbiota and its metabolites can enhance the effectiveness of immunotherapy, and certain gut microbiota can serve as biomarkers for predicting immunotherapy responses. Interventions targeting the gut microbiota, such as fecal microbiota transplantation, probiotics, prebiotics, engineered bacteria, and dietary interventions, have shown promise in improving the efficacy of ICIs and enhancing immunotherapy outcomes. The gut microbiota consists of bacteria, fungi, viruses, archaea, and potentially protozoa, and it interacts with host genes to modulate growth processes and adapt to environmental exposures. It plays a vital role in maintaining systemic immune homeostasis during bacterial infections and is integral to various immune processes. The intestinal mucosal barrier, composed of tight junction proteins, serves as a defense mechanism to separate bacteria from host cells. Alterations in the epithelial membrane can increase susceptibility to infections and facilitate the transfer of microbial metabolites to the host. In colorectal cancer (CRC), patterns of imbalance or disturbance in the gut microbiota have been recognized as indicators of specific diseases or poor health. Bacterial dysbiosis, impaired intestinal mucosal barrier, and cascade signaling pathways are key factors in CRC development. Bacterial toxins, such as those produced by *Clostridium perfringens* and *Fusobacterium nucleatum*, play a pivotal role in the development and progression of CRC. Inflammatory responses induced by the gut microbiota can contribute to the initiation and progression of CRC. The gut microbiota also affects other digestive tumors, including gastric cancer, hepatocellular carcinoma (HCC), and pancreatic ductal adenocarcinoma (PDAC). Short-chain fatty acids (SCFAs) and bile acids are important metabolites produced by the gut microbiota that contribute to improved gut health and regulate host metabolism. The gut microbiota and its metabolites have complex and diverse effects on the immune system, influencing both innate and adaptive immunity. In the tumor microenvironment, the gut microbiota impacts tumor progression through its metabolites, genotoxins, and signaling pathways. Gut microbiota composition can influence the response to ICIs, and specific gut microbiota can enhance the efficacy of immunotherapy. Interventions targeting the gut microbiota, such as fecal microbiota transplantation, probiotics, and prebiotics, have shown promise in improving the efficacy of ICIs and enhancing immunotherapy outcomes.