Spiders are thought to be strict predators. We describe a novel exception: Bagheera kiplingi, a Neotropical jumping spider (Salticidae) that exploits a well-studied ant-plant mutualism, is predominantly herbivorous. From behavioral field observations and stable-isotope analyses, we show that the main diet of this host-specific spider comprises specialized leaf tips (Beltian food bodies; Figure 1A) from Vachellia spp. ant-acacias (formerly Acacia spp.), structures traded for protection in the plant's coevolved mutualism with Pseudomyrmex spp. ants that inhabit its hollow thorns. This is the first report of a spider that feeds primarily and deliberately on plants.