Phenoloxidases (POs) are a family of enzymes including tyrosinases, catecholases and laccases, which play an important role in immune defences of various invertebrates. Whether or not laccase exists in shrimp and its function is still poorly understood. In this study, a laccase (LvLac) was cloned and identified from Litopenaeus vannamei for the first time. The full length of LvLac is 3406 bp, including a 2034 bp open reading frame (ORF) coding for a putative protein of 677 amino acids with a signal peptide of 33 aa. LvLac contains three Cu-oxidase domains with copper binding centers formed by 10 histidines, one cysteine and one methionine, respectively. Phylogenetic analysis revealed that LvLac was close to insects laccase 1 family. LvLac expression was most abundant in heart and the crude LvLac protein could catalyze the oxidation of hydroquinone. Real-time PCR showed that LvLac expression was responsive to Vibrio parahaemolyticus, Micrococcus lysodeikticus and white spot syndrome virus (WSSV) infection. Knockdown of LvLac enhanced the sensitivity of shrimps to V. parahaemolyticus and M. lysodeikticus challenge, suggesting that LvLac may play a positive role against bacterial pathogens.