Allochthonous inputs of basal resources have been shown to have profound effects on food web dynamics. Less is known about how subsidies of predators from neighboring habitats affect food web dynamics, including intraguild predation, prey suppression or trophic cascades. Here we provide evidence for one such predator subsidy that occurs annually in the salt marshes of North America. Upland reaches of the intertidal marsh are dominated by the cordgrass Spartina patens, with a well-developed litter layer, replaced at lower elevations by Spartina alterniflora, with less well-developed thatch. Anecdotal data suggest that S. patens acts as an overwintering refuge for the wolf spider Pardosa littoralis, whose populations proliferate annually into S. alterniflora where it can effectively suppress prey populations. We tested the hypothesis that Pardosa is restricted to the protective S. patens during winter, after which time its population proliferates into neighboring S. alterniflora. Results showed that in early spring, before reproduction, spider densities were much higher in S. patens than in S. alterniflora, confirming that S. patens provides refuge during winter. The majority of reproductive females remained in S. patens through early summer, but their offspring (young spiderlings) emigrated from S. patens to S. alterniflora. Pardosa density was negatively correlated with distance from S. patens over hundreds of meters, documenting the large spatial scale of this predator subsidy. As natural habitats become more fragmented, the opportunity for exchange of resources and organisms increases, and with it the importance of understanding the consequences of such subsidies on food web dynamics.