We studied the population-genetic structure of the Diamondback Terrapin (Malaclemys terrapin pileata) in Louisiana and used data from previous studies to compare our results with rangewide patterns of genetic diversity. We analyzed blood plasma to examine the contaminant load in Louisiana terrapins in the wake of the 2010 Deepwater Horizon oil spill. Using 16 polymorphic microsatellite loci, we tested for genetic structure to infer site fidelity, connectivity, and gene flow among terrapins in Louisiana estuaries. We found no evidence for population structure within Louisiana, even among sites up to 120 km apart. Comparing our Louisiana samples to data from previous studies using four loci, we found relatively low genetic diversity in Louisiana and other Gulf Coast terrapins. Based on their genetic similarity, our results support previous recommendations that two Gulf Coast subspecies be grouped into one management unit. The low genetic diversity we observed in Gulf Coast populations may be attributable to anthropogenic pressures, including massive overharvesting events in the early 20th century, from which populations have never fully recovered. Chemical analysis of plasma revealed low concentrations of contaminants, although analyses of other tissue types might have yielded a better estimate of oil contaminant sequestration for this species. Although our observed contaminant levels align roughly with levels of oiling at sampled sites, background levels as well as further analysis using more appropriate matrices (liver or fat) are needed to more accurately assess the impact of the 2010 Deepwater Horizon oil spill on the health of Gulf Coast terrapin populations.