Almost the whole life cycle of the grass puffer (Takifugu niphobles) occurs in seawater (SW), but it is also sometimes found in fresh water (FW) rivers. This study aims to evaluate the effects of FW exposure on the stress, osmoregulatory, and physiological responses of the grass puffer. The grass puffers were captured from a local wetland and acclimated to SW (35‰) or FW in the laboratory. In the stress responses, plasma glucose concentrations and the abundances of hepatic and branchial heat shock proteins were higher in the FW group than in the SW group. FW acclimation led to a significant increase in the protein abundance and the specific activity of branchial Na+/K+-ATPase (NKA). Immunochemical staining showed that the NKA immunoreactive (NKIR) cells of the FW and SW puffer were distributed mainly in gill filaments. Although the number of NKIR cells was similar in the two groups, the protein levels of proliferating cell nuclear antigen (PCNA) of nuclear fractions were elevated in the gills of the FW puffer. The induction of gill PCNA might contribute to cell proliferation which would maintain the amount of NKIR cells or repair DNA when exposed to FW, an osmotically stressful environment. Hence, activation of stress responses would provide the osmoprotection associated with FW adaptation of the grass puffer. Changes of branchial NKA expression and activity for osmoregulatory adjustment were required for stable blood osmolality and muscle water content. Based on our findings, the grass puffer was suggested to be a euryhaline teleost with SW preference. J. Exp. Zool. 319A: 409–421, 2013. © 2013 Wiley Periodicals, Inc.