The miragaia, Pogonias courbina, is an estuarine-dependent species that tolerates a wide range of salinity in its natural environment. This species is distributed along the Brazilian coast, from Rio de Janeiro to Rio Grande do Sul, also reaching the coasts of Uruguay and northern Argentina. Overfishing has placed the miragaia on the list of endangered species, highlighting the importance of understanding the effects of salinity on estuarine fish, especially those with potential for aquaculture and restocking, such as miragaia. The present study aimed to analyze the effects of salinity on: i) the ionic composition of blood plasma, including sodium, chloride, potassium, and calcium; and ii) the energy reserves of protein, lipid, and glycogen in the muscle and liver of Pogonias courbina juveniles. The juveniles (82 ± 12 g) used in this study originated from natural spawning of broodstock maintained at the Estuarine and Marine Fish Farming Laboratory of FURG. The individuals were exposed to five salinity levels: 2, 12, 22, 32, and 42‰ for 15 days, during which survival was 100% in all treatments. Analyses showed no significant differences in protein, lipid, and glycogen concentrations in the liver among the different salinities. In the muscle, no significant variations were observed in protein and lipid contents; however, glycogen concentration was significantly lower at 32‰ salinity. Isoionic concentrations of Na⁺, K⁺, Ca²⁺, and Cl⁻ were estimated as 222.2, 4.4, 1.7, and 139.9 mEq·L⁻¹, corresponding to salinities of 12.6, 15.6, 8.1, and 13.9‰, respectively. It is concluded that Pogonias courbina exhibits a broad ionoregulatory capacity, being a truly euryhaline species adapted to marked salinity variations.