Euryhaline fish, such as the Bullseye puffer Sphoeroides annulatus (Jenyns 1842), experience sudden salinity changes in their natural environment, which is more common than the exception, so they must adapt to survive and cope with extreme salt conditions. Therefore, Bullseye puffer juveniles were exposed to short-term stress (39 hr) by fluctuating salinity conditions (41, 35, 29, 23, 17, 11, 5, 11, 17, 23, 29, 35, 41 psu) with a 3-hr interval between each point at 26 ± 1°C in a respirometer chamber and acclimation reservoirs. Responses to oxygen consumption rate (OCR:oxygen-nitrogen atomic ratio (O:N 17-30), osmoregulatory pattern (blood osmotic pressure from 342.4 to 332.8 mmol/kg) and changes in expression levels of Na + /K + -ATPase in the gills (higher values at higher salinities) were measured. Although some signs of stress were detected below the iso-osmotic point (11.4 psu), the puffer fish is a strong euryhaline fish that survives under these conditions. Nonetheless, it could recover when salinity returned to the initial acclimation point because Sphoeroides annulatus is able to live in a wide range of environments with wide natural salinity fluctuations; thus, a common practice in aquaculture has been to expose fish to low salinity for several reasons discussed in this study. This capacity reveals its high plasticity to saline adaptation from 41 to 5 psu an up from 5 to 41 psu, all in less than 2 days. K E Y W O R D S