 We report the spillover of highly pathogenic avian influenza A, H5N1, into marine mammals in the northeastern United States, coincident with H5N1 and Sympactric Wild Birds. Our data indicate monitoring both wild coastal birds and marine mammals will be critical to determine pandemic potential of influenza A viruses. This article was authored by Wendy Perrier, Caitlin Sawatsky, Nicola Hill, and others.