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Hydrogen gas (H2) exerts a beneficial effect against traumatic brain injury (TBI). microRNA-21 (miR-21) is one of the most highly expressed members of small non-coding microRNA family in mammalian cells. miR-21 can improve the neurological outcome after TBI. In the present study, we investigated whether H2 treatment could improve the neurological outcome after TBI via increasing miR-21 expression. TBI was induced by controlled cortical impact in rats. H2 treatment was given by exposure to 2% H2 from 30 min to 5 h after TBI operation. Here, we found that H2 treatment significantly increased the expression of miR-21 in brain from 6 h to 3 d after TBI. The level of miR-21 expression in brain was significantly decreased after intracerebroventricular infusion of miR-21 antagomir in TBI-challenged rats with or without H2 treatment. Moreover, we found that H2 treatment conferred a better neurological outcome after TBI by improving neurological dysfunction, alleviating brain edema as well as decreasing lesion volume and blood–brain barrier permeability, which were significantly prevented by miR-21 antagomir. Furthermore, intracerebroventricular infusion of miR-21 agomir increased the level of miR-21 expression and decreased the lesion volume after TBI. In addition, H2 treatment decreased the levels of oxidative products (malondialdehyde and 8-iso-prostaglandin F2α) and increased the activities of endogenous antioxidant enzymes (superoxide dismutase and catalase) in brain after TBI, which were prevented by miR-21 antagomir. Taken together, these data indicate that H2 treatment improves the neurological outcome after TBI via increasing miR-21 expression.