We studied the sexual dimorphism, feeding habits, reproductive biology, and seasonal activity of the snake Philodryas patagoniensis in northeastern Argentina, where this species is the most abundant snake. There were differences in snout–vent length, tail length, and scale counts between adult males and females. Juveniles showed significant sexual dimorphism in tail length and scale counts. A total of 92 prey items was recorded from 184 specimens, with reptiles being the most common prey (58%) and Cercosaura schreibersii the most frequent individual prey item (26%). However, amphibians (15%), mammals (10%), and birds (4%) were taken occasionally. We found evidence of an ontogenetic diet shift because adults included endothermic prey. The female reproductive cycle was seasonal, and clutch size was positively correlated with female SVL. Philodryas patagoniensis was active throughout the year but significantly less so in winter and more so in spring. The great abundance of P. patagoniensis in South America may be influenced by its foraging and reproductive plasticity, feeding frequency, and the prey availability and abundance. Therefore, we suggest that this species may be an important predator in South American ecosystems.