A tumor is a heterogeneous aggregation of cells that undergo a large number of mutations. The study of tumor formation has led to the development of immunotherapies such as the inhibition of immune checkpoints PD-1 and CTLA-4. The biggest obstacle during tumor treatment is the development of resistance to existing immunotherapy. In order to overcome resistance in checkpoint immunotherapy, the attention of scientists is focused on intestinal microbiota as a potential modulator of immune response and therapeutic activity.The intestinal microbiota is a complex microbial community that is subjected to changes all throughout life. Some of its effects include maintaining the function of the gastrointestinal system and modulating the immune system through the production of metabolites or via direct interaction with immune cells. Disruption of the stability of the intestinal microbiota and the occurrence of dysbiosis is associated with a negative impact on health, while a stable intestinal microbiota exhibits an antitumor effect. The development of metagenomic analysis has enabled the investigation of intestinal microbiota as a potential modulator of the effectiveness of immunotherapy. Certain bacterial genera and metabolites are associated with eliminating resistance and improving response to immunotherapy through their stimulation of the immune system. Direct modulation of gut microbiota can be performed via faecal microbiota transplantation (FMT) which shows promising results in future tumor immunotherapy treatment. The exact mechanisms by which the intestinal microbiota effectsthe tumor microenvironment are not known, so further research is needed to define the exact influence of the intestinal microbiota and enable its clinical application.