We studied an offshore population of Clemmys guttata (Spotted Turtle) on a morainal island in Dukes County, MA, that served as a US Navy range from 1943 to 1996. We established a permanent study area consisting of 3 distinct wetland basins encompassing 2.11 ha (5.21 ac) of emergent marshes and ponds, including ∼4.3 % of all National Wetlands Inventory-classified palustrine wetlands on the island. Within our study area, we estimated the average population size of adult turtles to be 110.1 adult turtles during 2019–2021, or a density of 52.2 turtles/ha of wetland. Among the wetlands in our study area was a 1.1-ha (2.7-ac) emergent marsh 19 m (62 ft) in elevation that had breached into the ocean immediately prior to our study as the result of coastal erosion. Twenty-two adult turtles (28%) exhibited deformed or missing limbs, despite an absence of mammalian predators. The morainal islands of southern New England appear to be vulnerable to coastal erosion that can affect freshwater wetlands more than a dozen meters above sea level. Future work at this island should evaluate the response of Spotted Turtles—at the individual and population level—to ongoing coastal erosion, and the proximate causes of limb deformity.