The immune checkpoint inhibitors have opened new horizons in oncology. Although the indications for the use of Immune checkpoint inhibitors in cancer patients are expanding, there is still a need for markers that can aid in patient selection. Gastrointestinal microbiota can be among these markers. Recently, gastrointestinal microbiota stated to have a bidirectional relation with cancer immunotherapy with roles in both prognostic and therapeutic sides. Preclinical data suggest that modulation of the microbiota could become a novel strategy for improving the efficacy of immunotherapy. However, its labile structure prone to be affected by many factors. Further research can delineate the mechanisms of the relationship between microbiota and immunotherapy can have clinical implications.