As quantitative modeling of wildlife populations increases, the need for accurate and precise estimates of demographic rates for these populations also grows. Eastern Massasaugas (Sistrurus catenatus catenatus) are an imperiled rattlesnake species found mainly in the Great Lakes region of North America. We focused on an Eastern Massasauga population found on Beausoleil Island in Georgian Bay that was the subject of a 30-yr mark–recapture study for demographic analysis. We estimated multiple demographic values including annual adult survival, the temporal process variance of survival, population size, and population growth rate. Annual adult survival did not differ significantly between sexes (males 0.74; females 0.73). The process variance of annual adult survival for males was 0.006 and was inestimable for females. This is the first estimate of process variance for Eastern Massasaugas and one of the few such estimates for a snake species. The use of the process variance of survival in population prediction resulted in a large decrease in estimated extinction risk compared to use of the total variance of survival (3% and 21% extinction risk, respectively). Population size ranged from 35–77 (mean = 55), and realized population growth rate was equal to 1.02. Our analysis showed the Beausoleil Island population was stable up to the end of the study (2008). Demographic estimates can be used to guide management and conservation of this species.