Carnivorous mammals < 25 kg typically prey on species < 50% of their body mass but can choose prey whose energy value varies from small proportions of their daily needs to exceeding them. We hypothesized that for carnivores < 25 kg specializing in vertebrate prey, prey sizes closest to meeting daily energy needs would be most frequently depredated. We tested this hypothesis by reconstructing the diet of Humboldt martens using 528 scats and calculating the proportion of metabolizable energy (PME) that each prey taxon contributed to the diet. Overall, mammals dominated the diet (PME = 72%), followed by birds (PME = 22%), with berries, insects, and reptiles contributing < 10% PME. Sciurids comprised the largest proportion of all prey, representing 42% of overall PME, ranging from 29% (spring) to 51% (summer). While > 37 prey taxa were identified in the annual diet, only 11 contributed > 5% PME in any single season and the 4 dominant prey taxa in any single season represented 59–64% of that season's PME. Medium-sized prey (85–225 g) composed 55–66% PME from summer through winter and 2.6 to 8.4 times PME compared to small (< 40 g) and large (> 250 g) prey during these 3 seasons, respectively. When PME for the most frequently consumed individual medium-sized prey (e.g., chipmunks) declined seasonally, martens switched to alternative medium-sized prey (2.8- and 2.5-fold increases in medium-sized birds and flying squirrels, respectively), increased use of large prey (> 8-fold increase), but changed use of small prey least. The annual importance of medium-sized prey, and seasonal shifts to similar-sized or larger prey during winter-spring seasons, both support our hypothesis that the most frequently depredated prey in the diet of Humboldt martens have body sizes closest to meeting daily energy needs.