The discovery of immune checkpoint inhibitors (ICIs) has revolutionized the care of cancer patients. However, the response to ICI therapy exhibits substantial interindividual variability. Efforts have been directed to identify biomarkers that predict the clinical response to ICIs. In recent years, the gut microbiome has emerged as a critical player that influences the efficacy of immunotherapy. An increasing number of studies have suggested that the baseline composition of a patient's gut microbiota and its dysbiosis are correlated with the outcome of cancer immunotherapy. This review tackles the rapidly growing body of evidence evaluating the relationship between the gut microbiome and the response to ICI therapy. Additionally, this review highlights the impact of antibiotic-induced dysbiosis on ICI efficacy and discusses the possible therapeutic interventions to optimize the gut microbiota composition to augment immunotherapy efficacy.