Background Postoperative cognitive complications are associated with substantial morbidity and mortality. Ketamine has been suggested to have neuroprotective effects in various settings. This systematic review evaluates the effects of intraoperative ketamine administration on postoperative delirium and postoperative cognitive dysfunction (POCD). Methods Medline, Embase and Central were searched to 4 March 2018 without date or language restrictions. We considered randomised controlled trials (RCTs) comparing intraoperative ketamine administration versus no intervention in adults undergoing surgery under general anaesthesia. Primary outcomes were postoperative delirium and POCD. Non-cognitive adverse events, mortality and length of stay were considered as secondary outcomes. Data were independently extracted. The quality of the evidence (GRADE approach) was assessed following recommendations from the Cochrane collaboration. Risk ratios were calculated for binary outcomes, mean differences for continuous outcomes. We planned to explore the effects of age, specific anaesthesia regimen, depth of anaesthesia and intraoperative haemodynamic events through subgroup analyses. Results Six RCTs were included. The incidence of postoperative delirium did not differ between groups (4 trials, 557 patients, RR 0.83, 95% CI [0.25, 2.80]), but patients receiving ketamine seemed at lower risk of POCD (3 trials, 163 patients, RR 0.34, 95% CI [0.15, 0.73]). However, both analyses presented limitations. Therefore, the quality of the evidence (GRADE) was deemed low (postoperative delirium) and very low (POCD). Conclusion The effect of ketamine on postoperative delirium remains unclear but its administration may offer some protection towards POCD. Large, well-designed randomised trials are urgently needed to further clarify the efficacy of ketamine on neurocognitive outcomes.