Interleukin-21 (IL-21) is an important immune cytokine that was well characterized in human and mammals, but little is known in fish. In present study, an IL-21 homologue was cloned and well characterized from Tetraodon nigroviridis. The full-length Tetraodon IL-21 cDNA was 849 bp in size, containing an open reading frame (ORF) of 438 bp that translated a 145 amino-acid peptide, a 5’ untranslated region (UTR) of 69 bp, and a 3’ UTR of 342 bp. The deduced peptide shared identity of 20–49% with other known IL-21 sequences. The Tetraodon IL-21 gene had six exons while both human and Takifugu IL-21 gene contained only five exons. However, the level of synteny between human, Takifugu and Tetraodon genomes was well conserved during evolution. In vivo expression study showed that Tetraodon IL-21 mRNAs were constitutively expressed at a low level and only in limited tissues, including gut, gill and gonad in healthy fish, and stimulation with LPS increased the expression of IL-21 in these tissues and induced the expression of IL-21 in kidney, spleen and skin, indicating that IL-21 is an inflammatory stress inducible gene associated with the anti-bacterial defense in fish. Our study provided further evidence for the existence of IL-21 in fish, and gained further insight into the immunological functions of IL-21 gene in fish.