Objectives. There is emerging evidence for the association between the gut microbiome and psychological and cognitive symptoms. This systematic review examines this relationship in cancer patients treated with chemotherapy. Procedures. Seven electronic databases were systematically searched for articles measuring gut microbiome and psychological or cognitive symptoms in humans undergoing chemotherapy treatment for cancer. Studies that did not investigate the association between microbiome and symptoms were excluded. Data were extracted according to PRISMA guidelines. Quality of studies was assessed using QualSys (Kmet). Sample and Setting. Samples sizes ranged from 20-162 participants. 12 studies included adult participants who were mostly diagnosed with breast (n=4 studies) or rectal (n=4 studies) cancer. One study was conducted with children (7-18 years), with mixed diagnoses. Results. Of 1,918 studies identified, 13 met inclusion criteria: 11 were observational and 2 were interventional. All were published within the last five years and were of acceptable or high quality. All studies used patient-reported outcome measures (PROMs) to assess psychological outcomes including fatigue, with 2 studies also using objective neuropsychological assessment of cognitive function. Incidence of psychological morbidity varied when it was reported: 34-70% experienced fatigue, 0-20% experienced depressive symptoms and 29-34% experienced severe cognitive symptoms. Findings for alpha and beta diversity differences in the microbiome were inconsistent, but all observational studies identified individual taxa that were significantly correlated with symptom severity (psychological, cognitive, fatigue) or significantly differed between symptom groups. The two intervention studies both found positive impacts of the selected intervention (postbiotics or probiotics) on neuropsychological outcomes. Conclusion and Clinical Implications. This review highlights an association between the gut microbiome and chemotherapy-associated psychological and cognitive symptoms. While findings are preliminary, interventions targeting the microbiome (e.g., probiotics) show promise. Clinically, gut-targeted approaches may offer novel strategies to prevent or reduce treatment-related psychological and cognitive side effects in cancer patients.