The demography of the northern watersnake, Nerodia sipedon sipedon (Linnaeus, 1758), was systematically studied over a 17-year period in a complex of artificial ponds on protected property in the northern Allegheny Mountains of southwestern Pennsylvania. Sexual maturity was reached by the age of three years in males and four years in females. Asymptotic body size was reached at 55 cm in five years in males and at 85 cm in seven years in females. Population size was estimated to be about 43 snakes per year, with most adult males ranging 50–59 cm in body size and adult females ranging 70–79 cm in body size. Annual survivorship, recapture probability, and population size all increased in the years after wetland renovation. Among first captures, male:female sex ratio was 0.29:1.00, and juveniles comprised 26.4% of the population. Most adults were approximately five years old, and recapture intervals indicated minimum maximum ages of eight years for both males (n = 1) and females (n = 2), with skeletochronology estimating maximum ages ranging 16–18 years among the oldest and largest females. Clutch sizes averaged 19 young. Two of the six swabbed females tested positive for Ophidiomyces but with low fungal loads. Many of the life history traits examined in our study population conformed to those of other northern watersnake populations at northern latitudes. However, our results were indicative of a large population subjected to strong selection against small individuals and conflicting rewards for increasing body size.