Demographic information from geographically isolated conspecific populations is important for understanding how a species is locally adapted, and can thus inform conservation decisions. Clemmys guttata (Spotted Turtle) is declining throughout its range in eastern North America due to habitat loss and fragmentation and collection of specimens for the pet trade. The objectives of our study were to describe the demography of a previously unstudied island population of Spotted Turtles and to make comparisons to conspecific mainland populations. We conducted mark-recapture surveys for turtles on a small (23.2-ha) island in eastern Georgian Bay, ON, Canada. Over seven sampling trips, 40 different turtles were captured 72 times: 23 females, 6 males, 10 juveniles, and 1 hatchling. Males had significantly larger straight-line carapace lengths and contour carapace lengths than females, whereas females had greater carapace heights than males. Adult females on the island were significantly smaller than females on the mainland. Density was estimated to be 1.7 turtles/ha for the entire island, and 21.4 turtles/ha in one wetland where turtles aggregated in spring. The adult sex ratio was significantly skewed in favor of females (1 male: 3.83 females). Our study provides information on the population ecology of Spotted Turtles in isolation, which is important for the creation of management plans for populations being fragmented by human activities.