Recent studies indicate that the composition of gut bacteria can influence the effectiveness of certain cancer immunotherapy drugs and that modulating the gut microbiome may expand the pool of patients benefiting from cancer immunotherapies. Checkpoint blockade therapy has been effective on several types of malignancies (e.g. melanoma, lung cancer, kidney cancer). However, the number of patients that do not respond, or only partially respond, to cancer immunotherapy is high. Recently, several human and mouse studies have shown that gut microbiome may be a significant determinant of the response to cancer immunotherapy. This review focuses on the recent advances in our understanding of the interaction between human gut microbiome and response to immunotherapy in cancer. The gut microbiome may serve as a theranostic biomarker, by acting both as a useful prognostic biomarker and a target in cancer therapy.