Immune defense is indispensible for insect survival. However, uncontrolled and excessive immune responses would be highly detrimental and energy-consuming processes. An insect cytokine, plasmatocyte-spreading peptide (PSP), induces hemocyte-spreading behavior as well as activating phenoloxidase (PO) in the beet armyworm, Spodoptera exigua. A hemocyte transcriptome of S. exigua contains a partial sequence of a putative PSP-binding protein (SePSP-BP). SePSP-BP was expressed in all developmental stages especially in hemocytes and fat body. A quantitative RT-PCR showed that the bacterial infection significantly up-regulated the expression level of SePSP-BP. A double-stranded RNA specific to SePSP-BP (dsRNASePSP-BP) was injected and suppressed SePSP-BP expression even in response to bacterial challenge. The larvae treated with dsRNASePSP-BPsuffered high mortality to infection of nonpathogenic bacteria and prolonged high PO activity after the immune challenge. These results suggest that SePSP-BP may play a role in suppressing immune responses as a negative controller