Female multiple mating selects for male adaptations that maximize fertilization success in a context of sperm competition. While male mating strategies usually reflect a trade-off between present and future reproduction, this trade-off is largely removed in systems where the maximum number of matings for males is very small. Selection may then favor extreme mechanisms of paternity protection that amount to a maximal investment in a single mating. Males in several arthropod taxa break off parts of their copulatory organs during mating, and it has frequently been suggested that mutilated males can thus secure their paternity. Nevertheless, such a mechanism has rarely been confirmed directly. Here we study the golden orb spider Nephila fenestrata, which has a mating system with potentially cannibalistic, polyandrous females, and males that are often functionally sterile after mating with one female only. We show that males in this species can indeed protect their paternity by obstructing the female's genital openings with fragments of their copulatory organs.