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Interleukin-15 (IL15) is a proinflammatory cytokine that could induce the production of inflammatory cytokines. In this study, the α chain of the IL15 receptor of <i>Epinephelus coioides</i> (Ec-IL15Rα), a natural regulator of IL15, was identified, and immune response functions of fish were determined and characterized. Ec-IL15Rα contains a 720 bp open reading frame that encodes 239 amino acids, including four typical conserved cysteine residues with a highly conserved sushi domain. Ec-IL15Rα is closely related to <i>Epinephelus lanceolatus</i> and is the most clustered with teleost. Subcellular localization studies showed that Ec-IL15Rα was situated in the cytoplasm and cell membrane. <i>Ec-IL15Rα</i> was detected in 11 tissues, with the highest expression in the liver and blood. Meanwhile, the <i>Ec-IL15Rα</i> transcriptional levels substantially increased in nine tissues after <i>Vibrio harveyi</i> infection. <i>Ec-IL15Rα</i> was significantly up-regulated in HKLs by ConA, PHA, LPS and poly I:C stimulation. In vitro analysis, the recombinant protein of rEc-IL15Rα stimulates HKL proliferation and <i>IL1R</i>, <i>IL6R</i>, <i>IL10</i>, and <i>IL16</i> expression. Challenge experiments revealed that IL15Rα protein showed an increase of 6.67–10% survival protection rate after <i>V. harveyi</i> infection. This study provides a better understanding of the immune protection of IL15Rα in vertebrate fish.