Gamma-interferon-inducible lysosomal thiol reductase (GILT) has been described as a key enzyme that facilitating the processing and presentation of major histocompatibility complex class II-restricted antigen in mammals. In this study, the first echinoderm GILT named StmGILT was identified from sea cucumber (Stichopus monotuberculatus). The StmGILT cDNA is 1529 bp in length, containing a 5′-untranslated region (UTR) of 87 bp, a 3′-UTR of 674 bp and an open reading frame (ORF) of 768 bp that encoding a protein of 255 amino acids with a deduced molecular weight of 27.82 kDa and a predicted isoelectric point of 4.73. The putative StmGILT protein possesses all the main characteristics of known GILT proteins, including a signature sequence, a reductase active site CXXC, twelve conserved cysteines, and two potential N-linked glycosylation sites. For the gene structure, StmGILT contains four exons separated by three introns. In the promoter region of StmGILT gene, an NF-κB binding site and an IFN-γ activation site were found. The thiol reductase activity of recombinant StmGILT protein was also demonstrated in this study. In addition, the highest level of mRNA expression was noticed in coelomocytes of S. monotuberculatus. In in vitro experiments performed in coelomocytes, the expression of StmGILT mRNA was significantly up-regulated by lipopolysaccharides (LPS), inactivated bacteria or polyriboinosinic polyribocytidylic acid [poly (I:C)] challenge, suggested that the sea cucumber GILT might play critical roles in the innate immune defending against bacterial and viral infections.