Insular populations of mammals have been shown to undergo drastic morphological changes relative to mainland counterparts, and these adaptations can provide insight into the evolutionary effects of predation and competition. Selection has been shown to favor more energetically efficient body plans in insular mammals, even when this entails the reduction of anti-predator defenses, but few studies have focused on morphological effects within the same species. Previous research has established that insular large mammals tend to reduce in body size, and that size reduction may not scale isometrically across all body parts. The brain has been a particular subject of interest due to its high energy requirements. Here, we report that an extant, dwarfed island population of black-tailed deer (Odocoileus hemionus columbianus) exhibits significantly reduced brain mass relative to body mass in comparison to their mainland conspecifics, with brain mass 4.9% smaller in the island population for a given body mass. Further, in the dwarfed population, orbital area was 4.1% smaller for a given body mass, but this reduction was not significant. Foramina magna reduced isometrically. In resource-limited insular environments, negative allometry of the brain is consistent with predictions of reduced investment in energetically costly organs. This study is, to our knowledge, the first to examine the morphological effects of insularity on brain size in two conspecific populations, and these findings suggest that selection toward reduced brain size may act relatively quickly after isolation.