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No FDA approved pharmacological therapy is available that would reduce cell death following traumatic brain injury (TBI). Dexmedetomidine (Dex) is a highly selective agonist of alpha-2 adrenergic receptors and has demonstrated neuroprotective effects in hippocampal slice cultures undergoing direct impact. However, no one has tested whether Dex, in addition to its sedative action, has neuroprotective effects in an animal model of TBI. Thus, in the present study, we investigated the effects of Dex on an animal model of TBI. Mice received different doses of Dex (1, 10, or 100 µg/kg bodyweight, n = 10 each group) or saline as control at 1 hour and 12 hours following TBI. The mice treated with Dex lost less cortical tissue than the control mice. Further analysis found that Dex treatment reduced cell death in the cortex and the hippocampus measured by Fluoro-Jade B (FJB) staining, prevented axonal degeneration detected by immunostaining with antibody against β-amyloid precursor protein (β-APP), and protected synapses from elimination with synaptophysin staining. Taken together, in an in vivo murine model of TBI, Dex at the dose of 100 µg/kg not only prevented tissue lesion and cell death, but also reduced axonal injury and synaptic degeneration caused by TBI.