Memantine is a non-competitive N-methyl-D-aspartate receptor antagonist approved by the European Union and the U.S. Food and Drug Administration for the treatment of dementia and Alzheimer’s disease. In recent years, reports have suggested memantine’s potential efficacy in alleviating symptoms associated with attention deficit hyperactivity disorder and autism spectrum disorder. Current research indicates that one aetiological factor in these neurodevelopmental disorders may involve dysregulation of excitation/inhibition signalling in the nervous system, linked to dysfunctions in neuron–glia interactions. This review presents findings published to date on the safety and clinical efficacy of memantine in attention deficit hyperactivity disorder and autism spectrum disorder. The available literature is currently limited, consisting primarily of single randomised controlled trials and open-label studies conducted in small patient groups. The review includes publications involving participants aged 17 and older. In these studies, memantine was administered at doses ranging from 5 to 20 mg per day, either as monotherapy or as an adjunct to stimulant medication. Memantine appears to be a well-tolerated drug with few side effects. Preliminary results regarding its clinical efficacy are promising but do not yet support firm conclusions. Through this review, the authors aim to highlight the need for further methodologically rigorous studies in larger patient groups and to propose potential directions for future research.