An outbreak of adenoviral disease occurred in 2 separate raptor collections in the United Kingdom during August and September of 2004, involving a Harris hawk (Parabuteo unicinctus), a Bengal eagle owl (Bubo bengalensis), and a Verreaux's eagle owl (Bubo lacteus). The cases were diagnosed by results of necropsy, histologic examination, and polymerase chain reaction (PCR). Although virus isolation and electron microscopy were unsuccessful in identifying adenovirus as the causative agent, PCR testing with consensus primers resulted in amplicons of specific size. DNA sequencing of the PCR products identified the detected virus as a new member of the genus Siadenovirus. To our knowledge, this is the first report of adenovirus infection in these avian species, and we propose that this virus be called “raptor adenovirus.”