The human gastrointestinal tract harbors trillions of microorganisms, including bacteria, fungi, and viruses, to form the gut microbiota. Cumulative evidence has demonstrated the critical impact of gut microbes on cancer immunity. In cancer, an altered gut microbiota enriched with pathogenic bacteria can actively promote immune evasion and disrupt antitumor immunity, thereby supporting tumor growth and survival. Conversely, beneficial commensal bacteria (e.g., Lactobacillus and Bifidobacterium) have emerged as therapeutic probiotics for cancer prevention and as adjuvants for cancer therapy. The gut microbiota is also closely linked to the efficacy of immunotherapy. This review summarizes the effects of pathogenic bacteria and beneficial commensals, including T cells, B cells, natural killer cells, innate lymphoid cells, and myeloid-derived suppress cells, on various innate and adaptive immune cell populations in cancer. It also explores the mechanisms by which the gut microbiota influences immunotherapy efficacy, such as the modulation of innate immune cells and CD8 T cells. Given its importance, an increasing number of studies have developed approaches to target the gut microbiota to improve immunotherapy outcomes and reduce immune-related adverse events. These strategies include antimicrobial intervention, probiotics, prebiotics/dietary modifications, microbial metabolites, phage therapy, and fecal microbiota transplantation. This review also evaluates clinical applications that use the gut microbiota to predict immunotherapy outcomes. Overall, the current understanding of host‒microbe interactions within the tumor microenvironment has laid a critical foundation for the translation of microbiota research into clinical practice, ultimately benefiting patients.