Females of many species mate with multiple males even when it is costly. Multiple mating may allow females to exploit postcopulatory mechanisms to ensure that their eggs are fertilized by high quality (good genes) and/or genetically compatible males. We conducted a series of noncompetitive in vitro fertilization experiments to evaluate the benefits of polyandry in naturally occurring pairs of horseshoe crabs, Limulus polyphemus. In this externally fertilizing species, attached pairs migrate to shore to spawn; unpaired males are attracted to spawning pairs by visual and chemical cues and become satellites of some (polyandrous) females while ignoring others (monandrous). When present, satellites may fertilize a high proportion of the female’s eggs, but their presence is costly to female nesting success. We measured developmental success for monandrous and polyandrous females crossed with attached and satellite males. We found that satellite males increased the success of polyandrous but not monandrous females. We then examined the effect of good genes and genetic compatibility on developmental success and offspring size using a North Carolina II breeding design. Results indicate that both incompatibilities between males and females and paternal good genes effects may provide a selective advantage that offsets the costs of multiple mating in this species.