Gliomas are the most common and aggressive brain tumors, and a poor prognosis is correlated with its World Health Organization (WHO) grade. MicroRNAs (miRNAs) may serve as diagnostic and prognostic biomarkers in gliomas. In the present study, we collected plasma samples from patients with gliomas to evaluate the expression of miR-122 and analyzed the role of miR-122 in the diagnosis and prognosis of gliomas. We found that the expression of miR-122 in the plasma of patients with gliomas was significantly down-regulated compared to that in healthy individuals. In addition, the expression of miR-122, which was significantly correlated with WHO grade, decreased along with the development of gliomas. A receiver operating characteristic curve analysis showed high sensitivity and specificity of miR-122 for diagnosing gliomas (sensitivity 91.9%; specificity 81.1%; area under the curve 0.939). Finally, we found that lower expression of miR-122 was correlated with poor prognosis, and miR-122 was an independent prognostic parameter indicating poor prognosis for gliomas. In conclusion, our results showed that plasma miR-122 expression might act as a diagnostic and prognostic biomarker for gliomas.