Detailed studies on the natural history of snakes are essential for ecological hypothesis-driven research and effective conservation. Herein, we studied the diet composition and morphology of Yucatecan Cantils (Agkistrodon russeolus Gloyd 1972 [Serpentes: Viperidae]), an understudied viperid species endemic to the Yucatán Peninsula. We collected data from live free-ranging snakes; freshly road-killed snakes and snakes deliberately killed by local villagers; and museum specimens and literature sources. We compared their age classes, sexes, and color morphs. Data came from snakes from Yucatán, Quintana Roo, and Campeche in Mexico, and two records were from northern Belize, comprising most of the species' distribution range. Examination of 76 individuals resulted in 77 prey items from 68 snakes. Results indicated that A. russeolus is a generalist-opportunistic predator that feeds on a wide variety of vertebrates, but primarily on lizards and small mammals and less frequently on birds, snakes, and anurans. Geographic patterns in the diet and morphology of A. russeolus were evident: snakes from the coastal region were smaller in body size and less heavy bodied and with a heavily lizard-based diet, whereas snakes from both dry and moist forest regions were larger and heavier and their diet was primarily mammal based. We did not detect differences in diet composition among age classes because lizards and mammals were eaten at all stages of life, but an “ontogenetic telescope” was evident; that is, the importance of mammals increases with snake body size and the spectrum of prey types consumed also broadens. Males had longer tails than females, and females were light colored more often than males, but other morphological characteristics were similar between the sexes. The sexes and color morphs did not differ in diet composition. We did not detect dietary composition variations between seasons, and our data indicate that this species can feed year-round opportunistically. Because of increasing environmental pressures within its range, studies are urgently needed to elucidate other critical aspects of the ecology of this viperid species.