The Norway Rat (Rattus norvegicus) was a major predator on hatchling and juvenile Diamondback Terrapins (Malaclemys terrapin) in a New York population during three years (2001–2003). Rats killed young terrapins by evisceration through the plastron or carapace, exclusively at night, and during two distinct periods: (1) at emergence from nests in August and September; and (2) at emergence from hibernation in April. Predation rates were highest during peak emergence from nests and hibernacula. In the fall, hatchlings were mainly preyed upon within intertidal high marsh vegetation, where hatchlings normally occur after emergence from nests. We found no evidence of rat predation on eggs or hatchlings in nests. Predation in the spring also occurred in the intertidal high marsh, but rats killed juveniles in adjacent terrestrial habitats as well. We used data from a telemetry study of 24 wild hatchlings to estimate rat predation rates. Between 13 September and 22 October 2003, 16 of 24 (67%) radio-tracked hatchlings were killed by rats.