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Objectives: Observational and experimental studies suggest that the use of antibiotics close to administration of immune checkpoint inhibitors (ICI) can have a negative effect on tumour response and patient survival, due to microbiome dysbiosis and the resultant suppression of host immune response against neoplastic cells. Methods: A systematic search of PUBMED and EMBASE was undertaken for studies published between 1 January 2017 and 1 June 2020, evaluating the association between the use of antibiotics and clinical outcomes in patients with cancer treated with ICIs. A meta-analysis of the association between the use of antibiotics and clinical outcomes was also performed. Results: Forty-eight studies met the inclusion criteria (12,794 patients). Use of antibiotics was associated with shorter overall survival [hazard ratio (HR) 1.88, 95% confidence interval (CI) 1.59–2.22; adjusted HR 1.87, 95% CI 1.55–2.25] and progression-free survival (HR 1.52, 95% CI 1.36–1.70; adjusted HR 1.93, 95% CI 1.59–2.36), decreased response rate [odds ratio (OR) 0.54, 95% CI 0.34–0.86] and more disease progression (OR 2.00, 95% CI 1.27–3.14). The negative association between the use of antibiotics and progression-free survival was stronger in patients with renal cell carcinoma or melanoma compared with lung cancer. Only antibiotic administration >1 month prior to ICI initiation was associated with increased disease progression. Heterogeneity was substantial for all outcomes. Conclusions: Recent use of antibiotics in patients with cancer treated with ICIs was associated with worse clinical outcomes. Such patients may benefit from dedicated antimicrobial stewardship programmes.