Megalaria allenae, a blue-gray to green-blue sterile sorediate crustose lichen containing atranorin, fumarprotocetraric acid and zeorin, is described as new to science. It is corticolous on deciduous trees and endemic to swamp forests of the Atlantic Coastal Plain of southeastern North America. As sexual reproductive structures are unknown in the species, the generic placement and relationship with M. pulverea were determined using molecular phylogenetic analyses of ITS and mtSSU sequence data.