The c-Jun N-terminal kinase (JNK) pathway is involved in cell stress and apoptosis. We tested the hypothesis that this pathway plays a role in traumatic brain injury (TBI) by assessing JNK activation in human brain tissues and in brains of mice subjected to controlled cortical impact brain injury. We also assessed the effects of specific inhibition of the JNK pathway by the cell-permeable JNK inhibitor peptide, D-JNKI1, on neurobehavioral function and posttraumatic cell loss in mice. The inhibitor was administered intraperitoneally 10 minutes after injury. The JNK pathway showed robust activation both in human contusion specimens and in injured cortex and hippocampi of TBI-injured mice, 1, 4, and 48 hours after injury. D-JNKI1 treatment significantly improved motor performance at 48 hours and 7 days after injury and reduced the contusion volume compared with saline treatment; the numbers of terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling-positive cells were significantly decreased in the hippocampi of injured mice 48 hours after treatment. Thus, because the JNK pathway is activated after human and experimental TBI and the inhibitor peptide D-JNKI1 affords significant neuroprotection and amelioration of neurobehavioral deficits after experimental TBI, therapeutic targeting of the JNK activation pathway may hold promise for future clinical applications.