Bergmann’s rule predicts that organisms at higher latitudes are larger than ones at lower latitudes. Here, we examine the body-size pattern of the Atlantic fiddler crab, Minuca (=Uca) pugnax, from salt marshes on the east coast of the United States across 12 degrees of latitude. We found that M. pugnax followed Bergmann’s rule and that, on average, crab carapace width increased by 0.5 mm per degree of latitude. Minuca pugnax body size also followed the temperature-size rule with body size inversely related to mean water temperature. Because an organism’s size influences its impact on an ecosystem, and Minuca pugnax is an ecosystem engineer that affects marsh functioning, the larger crabs at higher latitudes may have greater per-capita impacts on salt marshes than the smaller crabs at lower latitudes.