In mammals, disulfide isomerase associated 3, PDIA3, is a member of the endoplasmic reticulum (ER) stress proteins, which can be induced by oxidative stress; however, its role in relation to stress regulation is still unknown in fish. Here, we report the cloning of a coding region of PDIA3 from the Atlantic salmon. PDIA3 mRNA expression was evaluated in the liver of Atlantic salmon exposed to environmental hyperoxia stress and toxic perfluorooctane sulfonate (PFOS) exposure stress. The PDIA3 sequence contained two PDI-typical thioredoxin active sites of WCGHC and shared approximately 70% identity with mammalian PDIA3, and its mRNA was primarily expressed in the liver. PDIA3 was significantly increased in the liver of Atlantic salmon exposed to hyperoxic water during smoltification. Also Mn superoxide dismutase (Mn-SOD) and CCAAT/enhancer binding protein (C/EBP), other markers of oxidative stress, were upregulated by hyperoxia. Furthermore, PFOS exposure of hepatocytes resulted in elevated mRNA expression of PDIA3, Mn-SOD and C/EBPdelta as well as peroxisome proliferator-activated receptor gamma (PPARgamma). These results indicate a signaling connection between oxidative stress and ER stress. PDIA3 and C/EBPdelta may be valuable markers in fish for exposure and effect to environmental stress.