1. High-quality information on predator-prey relationships is fundamental in understanding food webs, community assembly and ecosystem functioning. Recent analytical advances have made it possible to develop new trait-based approaches to study trophic relationships and evaluate trait-matching between predators and prey. 2. Here we develop a novel analytical approach based on generalized linear mixed-effects models (GLMM) to test the importance of prey availability and to identify the set of prey traits that best explain the occurrence and number of prey in the predator’s diet. 3. We demonstrate the approach by using an extensive dataset on prey availability, prey traits, and gut content collected in all known populations of Vipera graeca, a little-known, endangered snake of alpine grasslands in the Pindos Mountains of the Balkan Peninsula.4. We show that V. graeca is a unique, venomous snake specialised on bush-crickets and grasshoppers (Orthoptera). Prey selection GLMMs showed that the ideal prey of V. graeca is abundant, large-bodied, has poor escape abilities (flightless, slow-moving, bad jumper) and prefers loose grasslands (as opposed to bare ground/rock or closed sward). Vipers restrict their feeding to periods of high Orthoptera abundance in the late summer and need to reach a certain body size to become able to catch large-sized prey. 5. Our analytical approach provides a framework for trait matching between predators and prey and unprecedented fine-scale information on the importance of prey traits in prey selection by a specialist predator. The narrow trophic niche of V. graeca likely increases the vulnerability of this cold-adapted snake to extinction.