Abstract: Objective To investigate the epidemiological characteristics of respiratory adenovirus (Adv) infection among hospitalized patients under 18 years of age with pneumonia in Yancheng City of Jiangsu Province from 2023 to 2024, and to provide a basis for the prevention and control of adenovirus infection. Methods Nasopharyngeal swab samples were collected from hospitalized patients under 18 years of age diagnosed with pneumonia at two national influenza sentinel hospitals in Yancheng City from February 2023 to June 2024. A respiratory pathogen nucleic acid detection kit was used to test 28 pathogens including influenza A virus, influenza B virus, respiratory syncytial virus, human rhinovirus, adenovirus, Mycoplasma pneumoniae, Chlamydia pneumoniae, human parainfluenza virus, human bocavirus, enterovirus, polyomavirus K1, polyomavirus WU, parainfluenza viruses 1-4, coronavirus 229E, coronavirus HKU1, Haemophilus influenzae, coronavirus OC43, Bordetella pertussis, Group A Streptococcus, Staphylococcus aureus, Streptococcus pneumoniae, Legionella pneumophila, coronavirus NL63, Klebsiella pneumoniae, and the novel coronavirus, to analyze patients’ mixed infections. Adenovirus-positive samples were subjected to polymerase chain reaction (PCR) amplification using Hexon and Fibre as target genes, and the products were analyzed by sequencing. Whole-genome sequencing analyses were performed on positive samples with Ct values less than 22. Results: Among 368 hospitalized pneumonia patients aged under 18 years of age in Yancheng City from February 2023 to July 2024, the majority were male (51.63%, 190/368) and preschool-aged (42.66%, 157/368) and primary school-aged (40.22%, 148/368) children. A total of 44 cases were tested positive for adenovirus, yielding a positivity rate of 11.96%. Single infections, dual infections, and multiple infections accounted for 61.36% (27/44), 36.36% (16/44), and 2.27% (1/44), respectively. From November 2023, the adenovirus positivity rate rose sharply, peaking at 51.61% in December. Following PCR amplification and sequencing, valid sequences were successfully obtained for 37 strains, comprising 36 Adv-3 strains and 1 Adv-C strain. Eight adenovirus-positive samples with Ct values below 22 were inoculated into Hep-2 cell lines for virus isolation, and whole-genome sequencing analyses were performed on the five successfully isolated strains. Phylogenetic analyses revealed that the Adv-3 strains collectively belonged to the same evolutionary clade. Whole-genome sequencing results indicated that four Adv-3 strains (HAdv-B3) showed no evidence of genetic recombination, whereas one Adv-C strain (HAdv-89) might have undergone genetic recombination. Conclusion The adenovirus positivity rate among hospitalized pneumonia patients aged under 18 years of age in Yancheng from 2023 to 2024 was slightly higher than that reported by previous studies. Adenovirus infection exhibited a pattern of high incidence in winter, with the Adv-3 type being the predominant pathogen; one Adv-C strain (HAdv-89) underwent genetic recombination. It is recommended to strengthen surveillance of respiratory adenovirus type Adv-3 and its variants during the winter season, promote the use of multiplex pathogen detection techniques, and explore the clinical application of an Adv-3 vaccine.