High pathogenicity avian influenza (HPAI) viruses of clade 2.3.4.4 H5Nx have been circulating in poultry and wild birds worldwide since 2014. In South Korea, after the first clade 2.3.4.4b H5N1 HPAI viruses were isolated from wild birds in October 2021, additional HPAIV outbreaks occurred in poultry farms until April 2022. In this study, we genetically characterized clade 2.3.4.4b H5N1 HPAIV isolates in 2021–2022 and examined the pathogenicity and transmissibility of A/mandarin duck/Korea/WA585/2021 (H5N1) (WA585/21) in chickens and ducks. Clade 2.3.4.4b H5N1 HPAI viruses caused 47 outbreaks in poultry farms and were also detected in multiple wild birds. Phylogenetic analysis of HA and NA genes indicated that Korean H5N1 HPAI isolates were closely related to Eurasian viruses isolated in 2021–2022. Four distinct genotypes of H5N1 HPAI viruses were identified in poultry, and the majority were also found in wild birds. WA585/21 inoculated chickens showed virulent pathogenicity with high mortality and transmission. Meanwhile, ducks infected with the virus showed no mortality but exhibited high rates of transmission and longer viral shedding than chickens, suggesting that they may play an important role as silent carriers. In conclusion, consideration of both genetic and pathogenic traits of H5N1 HPAI viruses is required for effective viral control.