Investigations into brain function and disease depend on the precise classification of neural cell types. Cells of the oligodendrocyte lineage differ greatly in their morphology, but accurate identification has thus far only been possible for oligodendrocyte progenitor cells and mature oligodendrocytes in humans. We find that breast carcinoma amplified sequence 1 (BCAS1) expression identifies an oligodendroglial subpopulation in the mouse and human brain. These cells are newly formed, myelinating oligodendrocytes that segregate from oligodendrocyte progenitor cells and mature oligodendrocytes and mark regions of active myelin formation in development and in the adult. We find that BCAS1+ oligodendrocytes are restricted to the fetal and early postnatal human white matter but remain in the cortical gray matter until old age. BCAS1+ oligodendrocytes are reformed after experimental demyelination and found in a proportion of chronic white matter lesions of patients with multiple sclerosis (MS) even in a subset of patients with advanced disease. Our work identifies a means to map ongoing myelin formation in health and disease and presents a potential cellular target for remyelination therapies in MS.