Salinity-driven variation in photosynthetic traits is assessed for the dominant Amazonian mangrove species Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle on the Ajuruteua Peninsula, northern Brazil. Using A-Ci curves, gas-exchange measurements, and leaf functional traits, we quantified CO2 assimilation (Asat, Amax), stomatal conductance, Rubisco carboxylation capacity (Vcmax), electron transport rate (Jmax), and specific leaf mass (Ma) across a brackish and a saline site. All species exhibited higher photosynthetic rates and conductance under lower salinity, with A. germinans showing the highest photosynthetic capacity at both sites. Vcmax and Jmax were positively correlated, indicating coordinated biochemical limitation of photosynthesis. Ma was highest in R. mangle but showed no consistent relationship with Vcmax. Overall, salinity is a primary constraint on mangrove carbon assimilation, shaping interspecific physiological strategies. These findings enhance understanding of mangrove carbon dynamics and support improved modeling of ecosystem responses to environmental change.