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Lysozyme plays a crucial role in the innate immune response against bacterial phagocytosis by hydrolyzing the peptidoglycan layer of the bacterial cell wall. In this study, we characterized a goose-type lysozyme gene (<i>TrLysG</i>) in Japanese pufferfish. It is made up of an ORF of 573 bp that encodes a polypeptide of 190 amino acids. TrLysG includes a characteristic bacterial soluble lytic transglycosylase (SLT) domain, which contains three catalytic residues (Glu⁷¹, Asp⁸⁴ and Asp⁹⁵) and a highly conserved GLMQ motif (Gly⁹⁰, Leu⁹¹, Met⁹² and Gln⁹³). Phylogenetic analyses revealed that TrLysG is clustered together with its counterparts from other teleost fishes. Furthermore, mRNA expression analyses showed that <i>TrLysG</i> was highly expressed in healthy mucosal tissues (intestines and gills), and considerably up-regulated in response to <i>Vibrio harveyi</i> infection in the intestines, gills, and liver. At pH 6 and 55 °C, the pure recombinant TrLysG (rTrLysG) exhibits optimum activity. It also displayed antimicrobial activity against three Gram-positive bacteria (<i>Streptococcus parauberis</i>, <i>Staphylococcus pasteuri</i> and <i>Staphylococcus epidermidis</i>) as well as five Gram-negative bacteria (<i>Shewanella</i>, <i>Aeromonas hydrophila</i>, <i>Escherichia coli</i>, <i>Vibrio parahaemolyticus</i> and <i>V. harveyi</i>). Our results highlighted the significant role of TrLysG in immune defense against invading pathogens, thereby contributing to the prevention and alleviation of disease spread in aquaculture.