Abstract: Predator–prey interactions occur within the context of relative prey abundance, the nutritional value of prey individuals, and their specific vulnerability to predation. Predation patterns can therefore be expected to vary between seasons, age and sex categories, with this variation important for understanding predator–prey ecology. We examined seasonal prey selection by snow leopards (Panthera uncia) on different age and sex categories of Siberian ibex (Capra sibirica) using data collected from 150 kill sites. These were analysed in relation to known ibex population composition using Jacobs’ index, which estimates the strength and direction of prey preference or avoidance. Although solitary felids typically select prey of similar or smaller size than themselves, both male and female snow leopards selected for prime-aged ibex males (>5 years old), despite these being at least twice their body mass. Female ibex, yearlings and medium-sized males, which are similar in size to snow leopards, were preyed upon less than expected. In spring, snow leopards increased their prey selection of females and newborn kids, and reduced their selection of prime males. Our results suggest that the vulnerability of ibex to snow leopard predation is related to their agility in negotiating steep mountain terrain. We propose that the much larger body size of prime-aged ibex males reduces their ability to evade snow leopard ambushes on mountain slopes. In spring, the switch to predation on kids likely relates to the ease of hunting because of their lower agility, with a similar explanation for increased predation on females in late gestation. This seasonal switching between different prey categories, and the snow leopard’s specialisation to hunt in steep mountainous terrain, may impose limits on sexual size dimorphism commonly seen in other large felids. In contrast, the vulnerability of prime-aged ibex males to snow leopard predation may reflect a sexually selected handicap, imposed by high sexual size dimorphism.