Cross-species transmission of avian influenza A viruses (AIV) to marine mammals sporadically occurs. However, confirmed AIV-associated mortality has been restricted to harbour seals (Phoca vitulina) in the United States. In March, 2014, increased mortality of harbour seals was observed on the Swedish west coast and by December, 444 dead seals had been reported. On April 16, 2014, a 12 year old male harbour seal with respiratory distress was euthanized and examined by necropsy. It suffered from severe bronchopneumonia and AIV and Escherichia coli were detected in the lungs. AIV was further characterized by PCR and identified as H10N7. Phylogenetic analyses showed it was most closely related to the Eurasian lineage of AIVs circulating in wild and domestic birds, suggesting initial transmission from an avian source. Investigation into the extent of this outbreak using tissue, swab and/or serum samples from harbour seals found dead (n = 20) and from the general population (n = 144) is on-going. To date, AIV (H10N7) has been detected in lungs of nine individuals collected from April 16 - Sept 7, 2014. Preliminary results suggest that a significantly larger proportion of the seal population was exposed to the virus than the proportion that died during the mortality event and secondary complications (e.g. bacterial pneumonia) may have contributed to a fatal outcome. To our knowledge, this is the first report of AIV in seals in Europe, the first time it has been associated with mass mortality in seals outside of the United States, and the first time that H10 has been identified in seals. Inter-species transmission of AIV to mammals has potential public health significance and monitoring of AIV circulation in marine mammals coupled with further investigations into the pathology, epidemiology and zoonotic potential of H10N7, as well as adaptation of H10N7 to seals are warranted.