Past researchers have often considered neighbors to be beneficial to territorial residents, particularly compared with non-neighbor conspecific competitors. However, neighbors have the potential to be costly to residents in terms of both defensive costs and lost resources. In this study, we assessed the relative costs of defending a mating territory against neighbors and non-neighbors for the dragonfly Perithemis tenera, comparing across males with different numbers of contiguous neighbors; we also examined the possibility that the presence of contiguous neighbors might reduce the detection of potential mates. When neighbors were present, residents experienced a greater total number of intrusions by males; this increase in intrusions was due to higher numbers of intrusions by neighbors, as the number of intrusions by non-neighbor males did not differ. Residents with immediately adjacent neighbors also made more sorties toward neighbors than did residents whose nearest neighbors’ territories were not immediately adjacent. Interestingly, although the number of visits by females did not vary with the presence of neighbors, residents with neighbors made fewer sorties toward females than did residents without neighbors. Our results suggest that defensive costs increased when neighbors were present, that residents with neighbors may have missed opportunities to acquire mates, and thus that living with neighbors can be costly in this species.