ABSTRACT: The cordgrass Spartina alterniflora Loisel is a foundation species critical to the establishment and maintenance of western Atlantic salt marshes. Although the factors regulating cordgrass growth along sheltered, fine-sediment shorelines have been exhaustively studied, less is known about the mechanisms that maintain cordgrass production in high-energy marshes characterized by sandy substrates. We investigated whether deposit-feeding fiddler crabs Uca pugilator Bosc and U. pugnax Smith can mediate local physical conditions and nutrient availability and stimulate cordgrass primary production on sandy marsh sediments. We experimentally removed fiddler crabs from 3 × 3 m plots in an exposed, sandy marsh in Wellfleet, Massachusetts, USA, and found that above- and belowground cordgrass biomass decreased by >53% and 50%, respectively, and above- and belowground nitrogen in cordgrass (g N m^–2) decreased by >47% and 45% in fiddler crab removal plots compared to controls. Fiddler crab removal did not measurably influence sediment edaphic parameters, such as redox potential or waterlogging, suggesting that fiddler crabs enhance cordgrass productivity through nutrient deposition on sandy sediments and thus may be a critical component in the successful establishment and maintenance of salt marshes that develop along high-energy shores.