Clade 2.3.4.4b H5N1, H5N6, and H5N9 high pathogenicity avian influenza viruses (HPAIVs) were detected in poultry and wild birds during the 2025-2026 winter season in Korea. During this period, 62 H5Nx HPAI outbreaks occurred in poultry, and 63 cases were reported in wild birds. Genome constellation analysis revealed substantial genotype diversity. Most H5N1 and H5N9 genotypes were reassortants containing gene segments derived from East Asian clade 2.3.4.4b H5N1 HPAIVs since 2022 and low-pathogenicity avian influenza viruses from wild birds. Notably, the European H5N1 genotype EA-2024-DI.2.1, which spread rapidly across Europe in 2025, and its reassortants were identified in Korea. The novel poultry genotypes were also detected in wild birds, supporting wild bird to poultry spillover, whereas one genotype detected in previous season was identified in small-scale poultry farms. H5N1 (C588) of major 25G2 genotype in poultry and H5N9 (C722) of N9-G2 genotype showed high infectivity in chickens with 103.3 and 103.2 EID50 of median lethal dose (LD50) values, respectively. The predominance of the 25G2 genotype in poultry may have been influenced by efficient infection and virus introduction into high-density poultry production areas, potentially facilitating horizontal transmission under field conditions. In young ducks, H5N1 caused rapid mortality, whereas H5N9 induced limited clinical signs without mortality. Both viruses were transmitted to all contact ducks, increasing the risk of transmission to poultry. Despite intensified surveillance and early detection efforts to limit virus spread, H5Nx HPAIV outbreaks occurred during the 2025-2026 winter season. These findings highlight the need for continued surveillance, together with rapid genomic and pathogenic characterization of newly introduced viruses.