The role of the gut microbiota, especially bacterial flora, in the pathogenesis of inflammatory bowel disease (IBD) is becoming clearer. Advances in gut microbiota analysis and the use of gnotobiotics models have underscored the importance of gut bacteria and their metabolites in the progression of IBD. Fecal microbiota transplantation has shown promise in clinical trials for ulcerative colitis started as Advanced Medical Care B in Japan, raising expectations for its outcomes. This review explores the gut microbiota's role in IBD, encompassing both current knowledge and future prospects.