A new coral, Blastozopsammia guerreroterion, from the mid-Cretaceous (upper Albian–lower Cenomanian) Mal Paso Formation of southwestern Mexico is the earliest known and the first Mesozoic colonial member of the Dendrophylliidae, an extant worldwide group. Blastozopsammia is characterized by a ramose corallum produced by extratentacular budding, dimorphic corallites, and branches with a continuous axial corallite sheathed in an orderly arrangement of radially arrayed secondary corallites and a robust layer of reticulate coenosteum. This growth form is similar to that of many modern reef-building species of Acropora Oken, 1815 and species of the nonreefal PetrophylliaConrad, 1855 (=ArchoheliaVaughan, 1919). Based on corallite configuration, growth form and analogy with Acropora, Blastozopsammia had a relatively high degree of colony integration and may have been zooxanthellate. The combination of an axial corallite with radially arrayed secondary corallites has been regarded as one of the morphological and ecological pinnacles of coral evolution, yet it is rare among the Scleractinia. This growth form has not been recognized in any Cenozoic dendrophylliids. Blastozopsammia is an evolutionary enigma because no ancestral lineage or closely related taxon has been identified. However, the most likely origin of colonial Dendrophylliidae is Jurassic or Early Cretaceous Actinacididae.