The coastal zone may provide important foraging opportunities for insular populations of terrestrial island mammals, allowing for expanded habitat use, increased dietary breadth, and locally higher population densities. We examined the use of sandy beach resources by the threatened island fox (Urocyon littoralis) on the California Channel Islands using surveys of potential prey, beach habitat attributes, and scat and stable isotope analysis. Island fox consumption of beach items, primarily intertidal talitrid amphipods (Megalorchestia spp.), varied with the abundance of these prey across sites. Distance-based linear modeling revealed that abundance of giant kelp (Macrocystis pyrifera) wrack, rather than beach physical attributes, explained the largest amount of variation in talitrid amphipod abundance and biomass across beaches.  Carbon and nitrogen stable isotope values of fox whisker segments suggests individual diet specialization at the site level.  Some foxes specialized on terrestrial foods, while others showed a sustained use of sandy beach resources, the importance of which varied over time. Abundant allochthonous marine resources on beaches, including inputs of giant kelp, may expand habitat use and diet breadth of the island fox, increasing population resilience during declines in terrestrial resources associated with climate variability and long-term climate change.