High mobility group box 1 (HMGB1) is a chromosomal protein in the nucleus and a potent extracellular proinflammatory cytokine, widely described in mammals, nevertheless, with scarce reports in fish. In this study, full open reading frame ofHMGB1agene fromPiaractus brachypomusis reported as well as its molecular characterization, including tissue gene expression. At predicted protein level,HMGB1ashowed similarities with its orthologs in teleosts and higher vertebrates. The relative gene expression ofHMGB1amRNA was measured in several tissues including the brain, where a differential expression appeared in brain regions, i.e., higher expression in the cerebellum and telencephalon. In addition, in an assay of sublethal exposure to chlorpyrifos, upregulation ofHMGB1awas detected in optic chiasm. Furthermore, in a traumatic brain injury model, upregulation ofHMGB1aexpression was evident 24 hours after lesion and remained higher up to 14 days. These findings suggest a role forHMGB1ain brain damage and its candidature as biomarker of brain injury; however, more studies are required to elucidate the functions ofHMGB1aand its regulation inP. brachypomus.