Interferon regulatory factors (IRFs) are transcription factors which play important roles in regulating the expression of type I interferons (IFNs) and IFN-stimulated genes. IRF9 is one of the IRF family gene members which belongs to the IRF4 subfamily. Mammalian IRF9 has been known to be involved in antiviral responses as the DNA sequence recognition subunit of IFN-stimulated gene factor 3 (ISGF3) complex. In fish, only a few studies investigated the characteristics of IRF9 and the role in IFN signaling. In this study, we identified the IRF9 gene from miiuy croaker (mmiIRF9) and studied its feature and function. Sequence analysis showed the similarity of mmiIRF9 and other fish IRF9 genes. Structural and syntenic analysis showed the conservatism in fish IRF9 genes. The result of expression analysis in normal tissues and infected tissues and macrophages showed that mmiIRF9 expressed in all tested normal tissues and up-regulated expression in liver, kidney and macrophages after stimulated with poly(I:C). Luciferase reporter assays demonstrated the mmiIRF9 can induced IFNα and IFNβ luciferase reporters and the cellular localization of mmiIRF9 was mainly distributed in the cytoplasm in Hela cells. Furthermore, the evolutionary analysis of IRF4 subfamily showed the IRF4 and IRF8 may be the most ancient and conservative genes in the evolution of this subfamily.