Importance: Despite ongoing intercontinental spread of clade 2.3.4.4b H5N1 highly pathogenic avian influenza (HPAI), surveillance data from Mongolia -- a critical junction between Central and East Asian migratory flyways -- remain scarce, leaving a gap in understanding how viruses move between northern breeding areas and southern wintering grounds.

Objective: We aimed to detect and genetically characterize HPAI viruses in wild bird populations in Mongolia and assess their relationship with viruses circulating in East Asia.

Methods: Active surveillance was conducted at Ganga Lake, Mongolia, in October 2024. A total of 352 wild bird fecal samples were collected and tested for AIVs. Viral isolates were subtyped, whole-genome sequenced, and phylogenetically analyzed. Host species was identified using c oxidase I barcoding.

Results: Sixteen AIVs were isolated, including one clade 2.3.4.4b H5N1 HPAI virus (M9-3) from whooper swan feces. Genomic analysis revealed M9-3 had > 99% similarity with Korean and Japanese H5N1 isolates reported in late 2024 and undergone recent reassortment and disseminated through migratory flyways.

Conclusions and relevance: This is the first detection of clade 2.3.4.4b H5N1 in Eastern Mongolia in a fecal sample during late autumn. These findings underscore Mongolia´s importance as a sentinel location for interregional HPAI dissemination.