There are eight species established for aviadenoviruses: Fowl adenovirus A-E, Goose adenovirus A, Falcon adenovirus A and Turkey adenovirus B. The aim of this study was to sequence and analyse the complete genomes of turkey adenovirus 4 (TAdV-4) and TAdV-5 (strain 1277BT) in addition to almost two-thirds of the genome of another TAdV-5 strain (strain D1648). By applying next-generation sequencing, the full genomes were found to be 42 940 and 43 686 bp and the G+C content was 48.5 and 51.6 mol% for TAdV-4 and TAdV-5, respectively. One fiber gene was identified in TAdV-4, whereas two fiber genes were found in TAdV-5. The genome organization of TAdV-4 resembled that of fowl adenovirus 5 (FAdV-5), but it had ORF1C near the left end of the genome. TAdV-4 also had five 123 bp tandem repeats followed by five 33 bp tandem repeats, but they occurred before and not after ORF8, as in several fowl adenoviruses. The genome organization of TAdV-5 was almost the same as that of FAdV-1 but with a possible difference in the splicing pattern of ORF11 and ORF26. Phylogenetic analyses and G+C content showed differences that seem to merit the establishment of two new species within the genus Aviadenovirus: Turkey adenovirus C (for TAdV-4) and Turkey adenovirus D (for TAdV-5). Our analyses suggest a common evolutionary origin of TAdV-5 and FAdV-1.