Stable isotopes are useful tools for studying species residency and movement in aquatic environments. Yet, many questions about their use still remain, mostly related to assumptions that should be experimentally validated, such as turnover rates and discrimination factors. Salinity is a key environmental variable that may influence turnover and discrimination factors, but its effects have not been tested. A controlled diet-shift experiment was conducted with whitemouth croakers (Micropogonias furnieri), a euryhaline species, to determine the turnover rates and diet-tissue discrimination of carbon (δ13C) and nitrogen (δ15N) stable isotopes in muscle tissue. Fish captured in Patos Lagoon, Brazil were distributed into three independent recirculating aquaculture systems with salinities adjusted to represent freshwater, estuarine and marine environments, and fed for 90 days on an isotopically distinct diet. Half-life estimates were longer for both isotopes in the freshwater group (δ13C = 25.6 days, and δ15N = 34.6 days) compared to the estuarine (δ13C = 18.7 days, and δ15N = 23.9 days) and marine treatments (δ13C = 17.7 days, and δ15N = 22.3 days). Overall, carbon isotopic turnover was mainly driven by catabolism, whereas growth was the main factor responsible for nitrogen turnover. Trophic discrimination factors (TDFs) for carbon isotope were similar between fresh and estuarine treatments (TDFfinal = 0.82 ± 0.54‰ and 0.88 ± 0.58‰, respectively), but higher in marine (TDFfinal = 1.44 ± 0.66‰). Hence, in addition to providing species-specific isotopic parameters of a widespread sciaenid in the western Atlantic Ocean crucial to elucidate residence time along salinity gradients, these findings provide lab-based evidence of salinity effects on diet-tissue discrimination of a euryhaline species.