Myelination is the insulating ensheathment of axons by oligodendrocytes to increase action potential propagation and provide metabolic support. GABAergic interneurons are a heterogeneous group of predominantly local, inhibitory cells. In schizophrenia, a major debilitating psychiatric disorder, both myelination as well as functioning of GABAergic interneurons have independently been suggested to be impaired. Whether GABAergic interneurons themselves show myelination, remains unknown. Here, we show that GABAergic myelination has been extensively observed throughout various brain areas and different species. This interneuron myelination appears highly specific for the subclass of fast-spiking, parvalbumin (PV)-expressing interneurons. Indeed, nearly every PV interneuron exhibits myelination on its proximal axons in the adult brain. Furthermore, PV interneuron myelination is dependent upon neuronal activity. Finally, axonal morphology is shown to be a major determinant for myelination of PV interneurons as well as other interneuron subclasses. Together, interneuron myelination presents a widespread novel phenomenon with interesting implications for brain functioning in health and disease.