The loss and modification of freshwater ecosystems has led to high rates of imperilment for freshwater species. The Giant Gartersnake (Thamnophis gigas) is among the species that have suffered declines in abundance and spatial distribution and is currently listed as a threatened species by the U.S. government and the State of California. Conservation and management of populations of T. gigas are hampered by a lack of information on its demography. Without estimates of demographic parameters, the status of the population is difficult to characterize, and identifying the parameters to target in management planning is problematic. We used capture–recapture data from two populations in the Great Central Valley of California to estimate annual survival probability. We also evaluated hypothesized causes of variation in survival probability among individuals and among years. Model-selection results for the population in the American Basin indicated that females have a higher survival probability than males and that survival probability and the amount of precipitation between 15 April and 15 May in a year are negatively correlated. Associations with other weather covariates were also supported, but the evidence was weaker. For the population in the Natomas Basin, the model-selection results indicated a positive association between survival probability and the body size of an individual (snout-to-vent length). There was also evidence that females have higher survival probabilities than males, but the support for this effect was weaker. This work fills gaps in our understanding of the demography of T. gigas by providing estimates of survival probability for males and age classes for which estimates were previously not available.