The imbalance between mitochondrial fusion and fission has been implicated in cerebral ischemia and several neurodegenerative diseases. However, the role of mitochondrial fission in traumatic brain injury (TBI) remains poorly understood. Mitochondrial fission is mediated by dynamin-related protein 1 (Drp1), which is highly expressed in the nervous system. In the present study, we investigated the changes in Drp1 expression in the ipsilateral hippocampus of rats after TBI and the effects of Mdivi-1 (a selective inhibitor of Drp1) as a post-insult treatment for TBI. Our findings showed that the protein levels of Drp1 were increased at 6 h and peaked at 12 h post-TBI, but we did not observe Drp1 phosphorylation at Ser616, Ser637, Ser40 or Ser44 during this process. We examined the effect of Mdivi-1 on trauma-induced brain damage in both vitro and vivo. In cells, Mdivi-1 significantly attenuated HO-induced mitochondrial membrane potential (MMP) dissipation in PC-12 cells. Three days of Mdivi-1 treatment significantly reduced the cortical lesion volume, blood-brain barrier permeability, brain edema and oxidative stress. Mdivi-1 reduced activated caspase-3 release in the cortical border zone and hippocampal dentate gyrus three days after TBI. Furthermore, treatment with Mdivi-1 for 4 weeks rescued neurogenesis in DG and attenuated hippocampal atrophy. Regarding behavioral outcomes, Mdivi-1-treated TBI rats showed a significant improvement in water maze acquisition and retention compared with the saline-treated TBI rats. Moreover, Mdivi-1 treatment reduced anxiety-like behavior in an open-field test. Our results support the notion that Mdivi-1 provides brain protection and improves the behavioral performance in TBI rats.