Instances of mutualism and commensalism between alpheid shrimp and other marine invertebrates and fish are common in tropical waters. In this study, we present evidence that the temperate alpheid big-clawed snapping shrimp (Alpheus heterochaelis) participates in a symbiosis with the black-clawed mud crab (Panopeus herbstii), which constructs and maintains burrows in salt marshes of mid-Atlantic states, U.S.A. We surveyed eight mid-Atlantic salt marshes and found that 11% of occupied crab burrows (n = 1042) also housed Alpheus. Feeding trials showed that whereas Panopeus readily consumes other marsh Crustacea (i.e., grass shrimp and fiddler crabs), it will not prey on snapping shrimp. Alpheus caged in the marsh without access to mud crab lairs could not construct burrows. These results suggest that alpheid shrimps, thought to be limited in distribution to unvegetated mudflats and oyster reefs, may expand their intertidal range by living commensally with Panopeus herbstii in salt marsh habitats.