Dendritic cells (DCs) are the most potent professional antigen presenting cells and inducers of T cell-mediated immunity. However, few specific markers of mature DCs (mDC) have been reported. A previous microarray analysis revealed expression of mDC-specific genes and identified Rsad2 (radical S-adenosyl methionine domain containing 2) as a candidate specific marker for DC maturation. Mouse bone marrow-derived DCs were transfected with Rsad2 siRNA and examined by flow cytometry, ELISA, western, and confocal microscopy. C57BL/6 mice received intravenously B16F10 cells to establish a pulmonary metastasis model. Tumor-bearing mice then received subcutaneously two injections of mDCs or Rsad2 knockdown DCs. The cytotoxic T lymphocyte (CTL) population was examined from splenocytes of DC-vaccinated mice by flow cytometry. Rsad2 was induced at high levels in LPS-stimulated mDCs and mDC function was markedly attenuated under conditions of Rsad2 knockdown. Moreover, Rsad2 was necessary for mDC maturation via the IRF7-mediated signaling pathway. The importance of Rsad2 was confirmed in an Rsad2 knockdown lung metastasis mouse model in which mDCs lost their antitumor efficacy. Data on the CTL population further supported the results as above. Taken together, Rsad2 was an obvious and specific marker necessary for DC maturation and these findings will be clearly helpful for further understanding of DC biology.