Epilepsy is a chronic neurological disorder that has social, cognitive, and psychological consequences to the patient. The effects of the ketogenic diet (KD) in children and adults with pharmacoresistant epilepsy on cognitive function were evaluated in this systematic review. The MEDLINE, Cochrane Library, Scopus, Web of Science, and LILACS databases were searched up to February 2021. From the 2973 records initially identified, 24 studies were included in the systematic review. These records were screened via PICO criteria, focusing on studies that evaluated the effects of KD on cognitive function of patients with pharmacoresistant epilepsy. Nineteen studies described improvements in cognitive function attributed to KD; improvements were not observed in 2 studies, but neither was aggravation. Contradictory results were reported in 3 studies, depending on the method used to assess cognition. At first glance, cognitive function appears to be associated with the number of seizures, diet effectiveness, amount of carbohydrate ingested, and antiseizure medication used. However, due to the diversity of methods used to assess cognitive function, especially self-perception of cognitive improvement by the patient, it was not possible to confirm this hypothesis. It was not possible to confirm if KD itself promotes improvements in cognitive function in patients with pharmacoresistant epilepsy. Certainly, more studies are needed with better methodological quality, larger and more homogeneous samples in relation to epileptic syndrome and clinical aspects of the disease, more rigid monitoring of adherence to the diet, and use of standardized tests for neuropsychological assessment. Systematic Review Registration: PROSPERO registration no. CRD42019129236