Five chimney swift fledglings died following a progressive loss of appetite and condition while being cared for by an experienced wildlife rehabilitator. All animals had severe necrotizing and heterophilic ventriculitis, with myriad epithelial cells characterized by karyomegaly with intranuclear inclusion bodies. Transmission electron microscopy showed distention of epithelial cell nuclei and chromatin peripheralization by nonenveloped, icosahedral, 75- to 85-nm-diameter virions. Degenerate nested PCR for a highly conserved region of the adenovirus DNA polymerase gene was positive. BLAST analysis of the amplicon sequence indicated the presence of a novel adenovirus, with 74% homology to Antarctic penguin adenoviruses and 72% homology to a bat adenovirus, at low query coverages of only 65% and 63%, respectively. BLAST analysis of the predicted amino acid sequence generated the highest scores for squamate adenoviruses at 100% query coverage. Based on phylogenetic analysis of the partial amino acid sequence of the DNA polymerase, the chimney swift virus was a novel adenovirus most closely related to the Atadenovirus genus. Using a probe based on the novel viral sequence, DNA in situ hybridization identified viral nucleic acid in the nucleus. While the tentatively named chimney swift adenovirus-1 (CsAdV-1) is so far classified with the Atadenoviruses, it is relatively divergent from other members of that genus and may represent the first identified member of a new genus of Adenoviruses.