We have investigated the protective effect of immunization of juvenile red seabream, Pagrus major, with DNA plasmids encoding the viral major capsid protein (MCP) and an open reading frame (ORF) containing a transmembrane domain against red seabream iridovirus (RSIV). The expression of the MHC class I transcript in the DNA-vaccinated fish was significantly upregulated at the 15th day post-vaccination and the relative level of expression was maintained until the 30th day post-vaccination. This pattern of expression was similar in fish vaccinated with a commercially prepared formalin-inactivated RSIV vaccine. In vaccine efficiency tests, the relative percentage survival (RPS) of fish receiving the DNA vaccines and their combination ranged from 42.8 to 71.4% in two experimental runs, and these were significantly different from the control groups. Our results clearly demonstrate that DNA vaccines are able to induce robust protection in fish against RSIV infection, and a cellular immune response as shown by the upregulation of the MHC class I transcript after vaccination, which may be associated with such protection.