A new avian species from the late early Eocene Fossil Butte Member of the Green River Formation is described based on a nearly complete postcranial skeleton. The new species, Vadaravis brownae, gen. et sp. nov., can be diagnosed by a unique combination of characters, including the following autapomorphies, which are unique among Aves: two (cranial and caudal) small and discrete pneumatopores on the lateral sides of the caudal-most thoracic centra; and a caudoventrally located pisiform process of the carpometacarpus that projects only weakly cranially. Phylogenetic analyses recover Vadaravis as a member of the waterbird assemblage (including, e.g., penguins, storks, pelicans), closely related to taxa traditionally placed within the avian order Ciconiiformes (storks, flamingos, herons, the hamerkop, ibises, and spoonbills). Additional morphological features and a phylogenetic analysis constrained by a recently recovered waterbird topology suggest close affinities between Vadaravis and Threskiornithidae. This new species represents the first proposed part of Ciconiiformes (and possibly stem-Threskiornithidae) in the Green River Formation of North America. Its discovery increases the known taxonomic and ecological diversity of this diverse fossil avifauna. Vadaravis also represents one of the oldest members of Ciconiiformes (and possibly stem-Threskiornithidae), and implies that additional lineages within the waterbird assemblage had diverged by the late early Eocene.