Recent evidences revealed that the alteration of microRNAs (miRNAs) might be associated with neuroplasticity induced by voluntary running wheel (RW) exercise in mice suffered from traumatic brain injury (TBI). In the present study, we explored the possible role of miR21 involved in the cognitive improvement following voluntary RW in TBI mice. Firstly, in situ hybridization and quantitative real-time PCR (qRT-PCR) were employed to determine the hippocampal expression and location of miR21 in TBI mice with or without spontaneous RW. Either miR21-mimics/plenti-miR21 or miR21-agomir/miR21-sponge was employed to regulate the miR21 expression in vivo and in vitro. Acquisition of spatial learning and memory retention was assessed by Morris Water Maze (MWM) test. Golgi stain was also performed to evaluate the alteration of hippocampal dendrite. Our finding confirmed that the elevated miR21 level in hippocampal post-TBI was significantly reduced by spontaneous RW. Overexpression of miR21 in TBI mice with spontaneous RW induced deteriorations in spatial learning and memory retention by significant decreases in the somata size and branch points of the hippocampus neurons. In vitro transduction with miR21 also reduced the neurite extension and the area of cultured hippocampal neuron. However, miR21 down-regulation reversed these effects. The present data strongly suggest that miR21 is an important molecule that has been involved in neuroprotection induced by voluntary RW exercise post-TBI.