How the H7N9 avian influenza virus gained the distinct ability to infect humans is unclear. Pigs are an important host in influenza virus ecology because they are susceptible to infection with both avian and human influenza viruses and are often involved in interspecies transmission. Here, we passaged one avian isolate and one human isolate in pigs to examine the mammalian host adaptation of the H7N9 virus. The avian virus replicated to a high titer after one passage, whereas the human isolate replicated poorly after three passages in pig lungs. Sequence analysis found nine substitutions in the HA, NA, M and NS segments of the avian isolate, which enhanced the binding affinity for human-type receptors. These results indicate that avian H7N9 influenza viruses can be easily adapted to pigs and that pigs may act as an important intermediate host for the reassortment and transmission of such novel viruses.