Myeloid differentiation factor 88 (MyD88) is a crucial adaptor protein in the Toll-like receptor (TLR) signaling pathway and plays an important role in signal transmission. In this study, the full-length cDNA sequence of a MyD88 gene from Mytilus coruscus (McMyD88-4) was cloned. Its cDNA sequence is 3 930 bp, with a 2 607 bp open reading frame encoding a protein of 868 amino acids. Amino acids at position of 13-109 are the dead domain, and amino acids at position of 347-481 are the Toll/interleukin-1 receptor (TIR) domain, which contains three highly conserved regions Box 1, Box 2 and Box3. The three-dimensional structure of McMyD88-4 contains six α-helix and four β-sheets. According to the comparison with known MyD88 amino acids sequence, the putative protein of McMyD88-4 was most similar to that of Crassostrea gigas, with the identity and similarity of 60% and 77% respectively. Then it was more similar to Patinopecten yessoensis, Argopecten irradians, Ruditapes philippinarum, with 40%-51% of identity and 58%-67% of similarity. The result of the phylogenetic tree showed that McMyD88-4 of M. coruscus firstly clustered with C. gigas and scallops, and then with Drosophila melanogaster, while vertebrates formed a separate cluster. qRT-PCR revealed that McMyD88-4 was expressed in all tissues and organs examined. The highest expression was found in the mantle and gill, while the lowest expression was found in haemocytes. After infection by Vibrio chagasii, the expression of McMyD88-4 was up-regulated sharply in immune-related tissues, and reached the peak at 3 h and 6 h, and the up-regulation level in digestive gland was significantly higher than those in gill and mantle. The present results showed that McMyD88-4 plays an important role during the immune response of M. coruscus against external pathogens, especially Vibrio infections.