BACKGROUND AND AIM: Inclusion body hepatitis (IBH) is an acute and economically significant disease in poultry, caused by fowl aviadenovirus (FAdV), particularly serotypes belonging to species D and E. In Thailand, outbreaks of IBH associated with FAdV have been sporadically reported since 2007, yet comprehensive molecular surveillance remains limited. This study aimed to detect, molecularly characterize, and phylogenetically analyze FAdV strains associated with IBH in commercial broiler and breeder chicken farms across four provinces in Thailand. MATERIALS AND METHODS: A total of 28 liver samples were collected from chickens exhibiting clinical signs of IBH in Kanchanaburi, Chonburi, Lopburi, and Songkhla Provinces between June and December 2024. Gross and histopathological examinations were conducted, followed by a polymerase chain reaction targeting the hexon gene. Six representative positive samples were subjected to DNA sequencing and phylogenetic analysis using MEGA 11 software. Comparative amino acid sequence analysis was also performed to evaluate potential strain divergence. RESULTS: All 28 samples tested positive for FAdV, with gross pathology revealing pale, friable, and hemorrhagic livers. Histopathological analysis confirmed multifocal hepatic necrosis with characteristic basophilic intranuclear inclusion bodies. Sequencing and phylogenetic analysis identified all isolates as FAdV species E, serotype 8b. The isolates shared 94.73%-100% nucleotide similarity with reference strains from China, Indonesia, and Turkey. Phylogenetic clustering revealed two distinct groups among the Thai isolates, associated with specific amino acid substitutions at positions 17, 19, 20, 22, and 37 of the hexon gene. CONCLUSION: This study represents the first report of FAdV-E serotype 8b as the causative agent of IBH outbreaks in multiple commercial broiler and breeder chicken farms in Thailand. The detection of two phylogenetically distinct groups suggests the concurrent circulation of genetically diverse strains, potentially linked to vertical transmission routes. These findings underscore the urgent need for molecular surveillance, vaccination strategies utilizing local strains, and enhanced biosecurity measures to mitigate the spread of FAdV in the Thai poultry industry.