Adenoviruses have been regularly detected in squamate reptiles; evidence of infection in chelonians is described much less frequently. The adenoviruses found in turtles and tortoises have been genetically diverse, and have included members of the genus Siadenovirus, a proposed testadenovirus genus, and, in a single case, an Atadenovirus. In this study, samples from 949 chelonians submitted to a diagnostic laboratory were screened for the presence of adenoviruses by polymerase chain reaction (PCR) targeting a portion of the DNA polymerase gene. Adenoviruses were detected in 22 (2.3%) chelonians of different species. Adenovirus-positive species included Hermann's tortoises (Testudo hermanni), spur-thighed tortoises (T. graeca), Horsfield's tortoises (T. horsfieldii), sliders (Trachemys spp.), box turtles (Terrapene spp.) and a black pond turtle (Geochlemys hamiltonii). Sequencing and phylogenetic analyses of the obtained PCR products revealed that the majority of the detected adenoviruses (72.7%) cluster with members of the proposed testadenovirus genus, while the rest (27.3%) cluster with the atadenoviruses. This study significantly expands the known host range of both the proposed testadenoviruses and the atadenoviruses in different chelonian species and families.