Acute and adverse effects of ketamine on cognitive functioning have been documented. No longitudinal study has examined whether cognitive deficits can be reversed following ketamine abstinence although it has been suggested in some cross-sectional studies. This study aimed to investigate the changes in cognitive functioning among ketamine users following a 12-week abstinence from ketamine.In this longitudinal study, 114 ketamine users completed clinical and cognitive assessments at both baseline and 12-week follow-up with the following instruments: Severity of Dependence Scale, Beck Depression Inventory (BDI), Anxiety Subscale of the Hospital Anxiety Depression Scale (HADSA), and a cognitive battery.BDI (p < 0.001) and HADSA (p = 0.044) scores were significantly reduced at the 12-week follow-up. Significant improvements were found in Wechsler Adult Intelligence Scale (Third edition) immediate recall (p < 0.001) and delayed recall (p < 0.001) on the Rey-Osterrieth Complex Figure Test, and in delayed recall (p < 0.001), and immediate recall (p = 0.001) on the Logical Memory component of the Wechsler Memory Scale (Third Edition) at the 12-week follow-up. Participants completed the Stroop Inference Test significantly faster (p < 0.001); and required fewer number of attempts (p < 0.001) and produced fewer perseverative errors (p < 0.001) on the Wisconsin Card Sorting Test at the 12-week follow-up.Chronic ketamine users' verbal and visual memory and executive functions improved after 12 weeks of ketamine abstinence.