Pollen limitation, when inadequate pollen receipt results in a plant setting fewer seeds and fruits, can reduce plant reproductive success and promote the evolution of self-fertilization as a mechanism of reproductive assurance. However, the effect of pollen limitation on the joint evolution of mating system and seed size is not known. Using an evolutionarily stable strategy (ESS) resource allocation model, we show that where moderate pollen limitation and strong inbreeding depression select for complete outcrossing, pollen limitation should also increase the optimal seed size. In contrast, pollen limitation should not affect the optimal seed size under complete selfing, in which case ovule fertilization is certain. Under intermediate conditions, a mixed mating system evolves if the probability of ovule fertilization declines as more ovules are produced, so that a selfed seed with inbreeding depression provides equal marginal fitness returns to a larger outcrossed seed that may result from pollen limitation. Under mixed mating, outcrossed seeds should be larger than selfed seeds, and pollen limitation should not affect the optimal size of either outcrossed or selfed seeds. Our results identify a novel pathway through which pollen limitation selects for mixed mating, and provide an adaptive explanation as to why selfed seeds are often smaller than outcrossed seeds.