Dioecious mating systems in angiosperms, in which individuals are unisexual, is hypothesized to arise when there is a substantial fitness benefit to sexual specialization relative to co-sexual individuals. However, physically separate sexes force pollen to disperse between individuals to fertilize female flowers and may make dioecious species vulnerable to Allee effects. Evidence for Allee effects in dioecious species is relatively uncommon, potentially because species that experience such reduced reproductive success would be expected to maintain a co-sexual mating system. I present some of the first evidence for widespread reproductive failure in marine angiosperms using two species of the dioecious seagrass genus Phyllospadix. I demonstrate that male rarity (0 to 20% male) and localized pollen dispersal (max ~ 10-20m) leads to patchy pollination success and between population variation in seed set. Seed set estimates indicate that at some sites as few as 1 in 1000 ovules mature into seeds. Estimates of flower fertilization rates are positively correlated with the sex ratio, indicating that reduced male abundance negatively affects seed set. To assess whether this reduction in seed production impacts populations of Phyllospadix, which also reproduce asexually, I develop a simple size-based demographic model. Modeling results indicate that despite the presence of asexual reproduction, reduced seed production from male rarity has a substantial demographic impact on Phyllospadix. Male rarity, therefore, leads to reduced female fitness.