Many previous studies demonstrate that carbon stable isotope composition (usually expressed as isotopic ratio, δ¹³C) can reflect the integrated response of physiological processes to environment, but little is known of the variation in δ¹³C of mangrove plants as affected by salinity, especially in response to long term saline environment. The variation in δ¹³C of three dominant mangroves growing in long-term saline environments in the field was assessed. Mature leaves of Avicennia marina (Forsk) Vierh., Kandelia candel (L.) Duce., and Aegiceras corniculatum Blanco. were collected at seven sample sites covering seven salinity levels (14.07%–19.83%) at a natural mangrove wetland located in southeast China. The δ¹³C values varied from −31.46% to −27.33‰ with increasing salinity, reflecting the typical C₃ pathway of CO₂ fixation. The δ¹³C was positively related to salinity in K. candel and A. marina, and the reverse was observed in A. corniculatum, even though the correlations were not statistically significant for A. corniculatum and A. marina. The mean δ¹³C at high salinity was, in descending order, K. candel > A. marina > A. corniculatum. It is concluded that responses of δ¹³C to salinity is species-dependent and assessment of carbon isotopic composition is helpful for evaluating the salt tolerance of mangrove species, but it should be used with caution when comparing the salt tolerance between different species, particularly when they have the different patterns of δ¹³C in response to salinity.