Background: Evidence suggests that the human gut microbiota modulates the treatment response of immune checkpoint inhibitors (ICI) in cancer. Thus, finding predictive biomarkers in the fecal gut microbiota of patients who are less likely to respond to ICI would be valuable. This systematic review aimed to investigate the association between fecal gut microbiota composition and ICI-treatment response in patients with cancer. Methods: EMBASE, Medline, and Cochrane Library databases were searched using the "Participants, Interventions, Comparisons, and Outcomes" (PICO) process to locate studies including participants with solid cancers treated with ICI intervention. The comparator was the gut microbiota, and the outcomes were oncological outcomes such as response rates and progression-free survival. Study data were synthesized qualitatively in a systematic narrative synthesis, and the risk of bias in the studies was assessed. Results: Two reviewers screened 2092 abstracts independently, and 140 studies were read as full-text reports and assessed for eligibility. Eighteen studies were included with 775 patients with different types of solid cancers who received anti-PD-1, anti-PD-L1, or anti-CTLA-4 therapy. Distinct patterns were observed in the patients' fecal samples. Some bacterial species were reported to be present in responders and non-responders, while others were present only in one group. The most reported species associated with better prognosis were Faecalibacterium prausnitzii, Streptococcus parasanguinis, Bacteroides caccae, and Prevotella copri. In contrast, the most reported species associated with poor prognosis were Blautia obeum and Bacteroides ovatus. Conclusion: Distinct microbiota features were associated with good and poor prognoses in ICI-treated patients with cancer.