Host-specific natural enemies can increases diversity of their prey or hosts by preventing single-species dominance (the Janzen-Connell hypothesis). Here we postulate a “reverse Janzen-Connell effect” mediated by mutualists. We hypothesise that mutualistic ants, acting as natural enemies of herbivores, reduce conspecific negative density dependence (CNDD) in plants by protecting nearby conspecific seedlings from herbivory. We tested this on New Guinean ant-plants using: i) seedling growth and survival rate measurements, ii) spatial pattern analyses of ant-plants, and iii) seedling translocations . We found more rapid growth and lower mortality for seedlings of some ant-plant species and weakened CNDD for ant-plants in the family Meliaceae. Transplanted seedlings from which patrolling ants were excluded experienced increased herbivory, slower growth, and greater mortality than controls. Our results indicate that mutualists can weaken negative density dependence, and hence could reduce host species diversity. We speculate that reverse Janzen-Connell effects could influence species richness across a wide range of systems.