In many animals females mate with multiple males during a single breeding season (polyandry), but the benefits of this mating system remain poorly understood. One hypothesis is that polyandry ensures the fertilization of a female's ova (fertilization insurance hypothesis). We tested the fertilization insurance hypothesis in a natural population of African foam-nesting treefrogs, Chiromantis xerampelina, a species lacking male contest competition and in which females routinely mate with multiple males. We observed matings involving from one to 12 males and found that fertilization success was positively correlated with the number of mating males, but was unaffected by variance in clutch size or ambient temperature. Variance in fertilization success was also unrelated to the body size ratio of mating pairs. Critically, females that mated with more males also produced more tadpoles. These findings provide the first evidence that polyandry can benefit female frogs by increasing fertilization success and offspring production. We propose that fertilization insurance may account for the high incidence of simultaneous polyandry in frog species that use foam nests during breeding.