In the mature CNS, netrin-1 is expressed by neurons and oligodendrocytes and implicated in the stability of axo-oligodendroglial paranodal junctions. Here we report that the netrin receptor UNC5B is highly expressed by mature oligodendrocytes and enriched at paranodes. We demonstrate that paranodes become disorganized following conditional deletion of UNC5B in oligodendrocytes, with disruption of the interface between glial loops and detachment of loops from the axon. As a result, Caspr1 and Kv1.1 disperse along the axon, internodes fail to lengthen and compact myelin periodicity is reduced. Paranodal and axoglial domain disorganization progressively worsens and a delay in motor learning develops in aged mice lacking oligodendroglial UNC5B. Altered glial loop ultrastructure and reduced levels of claudin-11 and JAM-C tight junction proteins support the conclusion that disruption of autotypic junctions between paranodal loops underlies paranode disorganization. Our findings reveal an essential contribution of oligodendroglial UNC5B at paranodes that is required for the stability of mature myelin. ### Competing Interest Statement The authors have declared no competing interest.

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