Seasonal shifts in vegetation-masting events may alter resource availability and influence habitat selection. Crotalus adamanteus (Eastern Damondback Rattlesnake; hereafter, EDB) is an imperiled, ambush predator endemic to southeastern pine savannas and woodlands of the US. Eastern Diamondback Rattlesnakes prey on small mammals that feed on hard and soft mast (e.g., nuts and fruits). In this study, we hypothesized that intra-seasonal shifts in masting vegetation would cause intra-season shifts in ambush-site selection in EDBs as the result of a bottom-up trophic effect. We quantified EDB ambush-site selection using radio-telemetry data and vegetation analysis within a naturalized study site. When we encountered EDBs in ambush posture, we quantified vegetation structure at the selected location and at 2 random locations. We measured understory and overstory structure and masting characteristics within each vegetation plot. Over the June–August study period, we quantified vegetation structure at 35 ambush sites and 70 paired random locations. We used conditional logistic regression to model ambush-site selection. We constructed 5 a priori models to examine ambush-site selection, with soft-mast presence, hard-mast presence, and canopy cover as predictors. The top models supported our hypothesis, indicating a significant association with soft-mast—producing vegetation during times when soft mast was present. Hard-mast presence was also an important predictor of EDB ambush sites. The results of this study indicate that EDB foraging-site selection reflects mast availability, which may be an indication of a bottom-up trophic effect. We should consider mast presence and absence in efforts to manage EDB populations and their prey.