Interleukin-17 (IL-17 or IL-17A) production is a hallmark of T(H)17 cells, a new unique lineage of CD4(+) T lymphocytes contributing to the pathogenesis of multiple autoimmune and inflammatory diseases. IL-17 receptor (IL-17R or IL-17RA) is essential for IL-17 biological activity. Emerging data suggest that the formation of a heteromeric and/or homomeric receptor complex is required for IL-17 signaling. Here we show that the orphan receptor IL-17RD (Sef, similar expression to FGF genes or IL-17RLM) is associated and colocalized with IL-17R. Importantly, IL-17RD mediates IL-17 signaling, as evaluated using a luciferase reporter driven by the native promoter of 24p3, an IL-17 target gene. In addition, an IL-17RD mutant lacking the intracellular domain dominant-negatively suppresses IL-17R-mediated IL-17 signaling. Moreover, IL-17RD as well as IL-17R is associated with TRAF6, an IL-17R downstream molecule. These results indicate that IL-17RD is a part of the IL-17 receptor signaling complex, therefore providing novel evidence for IL-17 signaling through a heteromeric and/or homomeric receptor complex.