Inflammation is a prominent feature of CNS injury that heavily influences neuronal survival, yet the signals that initiate and control it remain poorly understood. Here we identify the nuclear alarmin, interleukin (IL)-33, as an important regulator of theinnate immune response after CNS injury. IL-33 is expressed widely throughout the healthy brain and is concentrated in white mater due to predominant expression in post-mitotic oligodendrocytes. IL-33 is released immediately after CNS injury fromdamaged oligodendrocytes, acting on local astrocytes and microglia to induce chemokines critical for monocyte recruitment. Mice lacking IL-33 haveimpaired recovery after CNS injury, which isassociated with reduced myeloid cell infiltrates anddecreased induction of M2 genes at the injury site. These results demonstrate a novel molecular mediator contributing to immune cell recruitment tothe injured CNS and may lead to new therapeuticinsights in CNS injury and neurodegenerative diseases.