Ants had been absent from the Hawaiian Islands before their human introduction. Today they cause severe alterations of ecosystems and displace native biota. Due to their strong demand on carbohydrate-rich resources, they often exploit floral nectar of native Hawaiian plant species with largely unknown consequences for the plants’ reproduction. We examined effects of flower-visiting invasive ants on reproduction of the endemic shrub Vaccinium reticulatum (Ericaceae) in Hawai‘i Volcanoes National Park. Ant densities in flowers were high and floral nectar was excessively exploited, which may lead to a reduced visitation rate of pollinators. However, the ants’ presence on flowers strongly reduced flower parasitism by caterpillars of the introduced plume moth Stenoptilodes littoralis and thus decreased the loss of flowers and buds. This is, to our knowledge, the first documented mutualism between invasive ants and an endemic plant species in Hawai‘i. Developed fruits of this partly self-incompatible plant, however, bore relatively low proportions of viable seeds, irrespective of the experimentally controlled visitor spectrum of the flowers. This may indicate that ants do not function as pollinators and that effective pollinators (probably Hylaeus bees) are scant or absent.