In January 2021, an outbreak of H5N8 subtype avian influenza was identified from wild mute swans (Cygnus olor) in an overwintering site located within Yellow River Delta Nature Reserve in Shandong, China for the first time. We sequenced the HA and NA genome segments of the virus, named as A/mute swan/Shandong/1/2021 (H5N8) and explored the genetic characterization and comparative phylogenetic analysis to track its origin. The wild swans prior to death showed nervous signs accompanied with twisted neck. The major pathological changes of the heart, lung, trachea, kidney, spleen, and glandular stomach were hemorrhage, and extensive necrotic lesions in the liver and pancreas were also observed. Further analysis of the HA and NA phylogenetic trees generated by H5N8 strains isolated worldwide revealed that the isolate have a closely genetic relationship with H5N8 subtype AIV from recently isolated in Korea and Japan from 2020 to 2021, suggesting the potential role of mute swans in the global dissemination of H5N8 subtype AIVs along the migration routes. Therefore, enhanced active surveillance in wild and domestic birds is required to monitor the introduction and spread of H5N8 subtype AIV by migratory birds in order to decrease the risk of H5N8 outbreaks.