This study was initiated to provide baseline data and to determine the utility of stable isotope analysis to evaluate the foraging strategies of an opportunistic reptile predator. Stable isotope ratios of carbon and nitrogen were evaluated from multiple tissues from terrapin populations to determine spatial or temporal variations in resource use within mangrove habitats in Southern Florida. We sampled Diamondback terrapin (Malaclemys terrapin) and potential resources within mainland and island habitats, and evaluated their δ13C and δ15N values. We fit linear regression models to determine the best predictors of isotopic values for both terrapins and their prey, and SIBER analysis to examine terrapin isotopic niche space and overlap between groups. We identified differences in terrapin isotopic δ13C and δ15N values among all sites. Blood and scute tissues revealed different isotopic compositions and niche overlap between sites, suggesting diets or foraging locations may change over time, and amount of variation is site specific. Niche overlap between size classes was larger for blood (short-term) versus scute (long-term), suggesting greater variability in food resource use and/or isotopic signal of those food resources over short and long timescales.