Serotonin receptors, including ligand-gated ion channel (LGICs) and G protein-coupled receptors (GPCR), play vital roles in modulating physiological processes and immunoreaction. In the present study, a homologue of serotonin (5-HT) receptor was identified from oyster Crassostrea gigas (designated Cg5-HTR-1). Its open reading frame (ORF) was of 1239 bp, encoding a polypeptide of 412 amino acids with a seven transmembrane region. Cg5-HTR-1 shared high similarity with the 5-HTRs from other animals. The cAMP contents in HEK293T cells decreased significantly after Cg5-HTR-1 transfection and 5-HT incubation (p < .05), while blocking Cg5-HTR-1 with specific receptor antagonist reversed this downtrend. The intracellular Ca2+ concentrations increased significantly (p < .05) after cell transfection and 5-HT incubation, and the antagonist treatment also arrested this process. Cg5-HTR-1 transcripts were widely distributed in various tissues, with the highest level in hepatopancreas and lowest level in mantle and gill. The mRNA expression of Cg5-HTR-1 in hemocyte increased significantly after lipopolysaccharide (LPS) stimulation and reached the peak level (6.47-fold, p < .05) at 6 h post treatment. The inhibition of Cg5-HTR-1 significantly reduced the expression of tumor necrosis factor (TNF) mRNA in hemocyte, down-regulated the superoxide dismutase (SOD) activity in serum, and induced the apoptosis of hemocyte (p < .05). These results suggested that Cg5-HTR-1 was a novel member of 5-HT1 receptor family and it mediated serotonergic immunomodulation on both cellular and humoral immune responses.