To assess and analyse accurately their geographical distributions, fiddler crabs (Uca) from the Atlantic coast of Brazil were studied using field collections, museum specimens and literature reports. More than 7000 specimens of ten Uca species were collected across five coastal biomes from 63 sites on the Atlantic coast of South America between the Brazilian States of Amapá and Santa Catarina in 2009 and 2010. Based on our analyses of mouthparts and habitat osmolality (=salinity), each species seems to prefer a particular niche defined by a specific array of physical factors. The species' distributions form four geographical patterns along the South Atlantic coast; however, the different species do not occur uniformly within each biome. Most common are U. (M.) rapax, U. (L.) leptodactyla, U. (B.) thayeri and U. (U.) maracoani with the relative abundance of each varying among the biomes according to habitat availability. Owing to their high constituent diversity, two geographical areas are candidates for biological conservation efforts. Considering fossil records, molecular clocks, oceanic hydrology and plate tectonics over the last 65 million years, the biogeographical patterns of the extant species can be used to reconstruct an evolutionary scenario for Uca in the tropical and temperate zones of the western Atlantic Ocean.