Recent infections and transmissions of highly pathogenic avian influenza (HPAI) H5N1 clade 2.3.4.4b viruses in humans and animals pose a significant global public health threat. Vaccination is the most effective method for preventing and controlling human infections; however, it is unclear whether the World Health Organization (WHO) candidate vaccine viruses (CVVs) can induce cross-neutralizing antibodies against H5N1 strains currently circulating in the Republic of Korea. In this study, we evaluated sera from ferrets immunized with four WHO HPAI H5 CVVs (clade 1 and clade 2.3.4.4b) against HPAI H5N1 clade 2.3.4.4b viruses isolated from poultry and mammalian outbreaks in the Republic of Korea during 2023–2024. Hemagglutination inhibition and microneutralization assays showed that ferret antisera raised against the H5N1 clade 2.3.4.4b CVVs exhibited broad cross-neutralization against most Korean isolates. Among these, antiserum derived from the A/Ezo red fox/Hokkaido/1/2022 showed the strongest cross-activity. However, ferret antisera raised against the A/Vietnam/1194/2004-derived clade 1 CVV showed negligible neutralizing responses. These findings suggest that H5N1 clade 2.3.4.4b CVVs are effective vaccine candidates for inducing cross-neutralizing antibodies and could play a critical role in preparedness efforts against potential human infections in the Republic of Korea.