Tumor necrosis factor receptor-associated factor 3 (TRAF3) is an important adaptor that transmits upstream activation signals to induce innate immune responses. TRAF3 interacting protein 1 (TRAF3IP1) interacts specifically with TRAF3, but its function in innate immunity remains unclear, especially in marine invertebrates. In this study, to better understand the functions of TRAFs in innate immune responses, we identified and characterized the first bivalve TRAF3IP1 gene, PyTRAF3IP1, from Yesso scallop (Patinopecten yessoensis), one of the most important mollusk species for aquaculture. The PyTRAF3IP1 cDNA is 2,367 bp, with an open reading frame of 1,629 bp encoding 542 amino acids. Phylogenetic and protein structural analysis confirmed the gene's identity and revealed that PyTRAF3IP1 was more similar to vertebrate TRAF3IP1s than to those of invertebrates. PyTRAF3IP1 was expressed in all the adult tissues and developmental stages sampled, implying that it plays versatile roles in many biological processes. Furthermore, PyTRAF3IP1 expression was dramatically induced in the acute phase (3-6 h) after infection with both Gram-positive (Micrococcus luteus) and Gram-negative (Vibrio anguillarum) bacteria, even stronger induction being observed after V. anguillarum challenge. This is the first report of the characterization and immune response involvement of TRAF3IP1 in marine invertebrates, and suggests that TRAF3IP1 contributes to innate immunity in bivalves.