The emergence of the human 2009 pandemic H1N1 (H1N1pdm) from swine populations re-focused public and scientific attention towards swine as an important source of influenza A viruses bearing zoonotic potential. Widespread and year-round circulation of at least four stable lineages of porcine influenza viruses in Germany in a region with a high density swine population between 2009 and 2012 is documented here. European avian-derived H1N1 (H1N1av) viruses dominated the epidemiology, followed by human-derived subtypes H1N2 and H3N2. H1N1pdm viruses and, in particular, recently emerged reassortants between H1N1pdm and porcine HxN2 viruses (H1pdmN2) were detected in about 8% of cases. Further reassortants between these main lineages were diagnosed sporadically. On-going diversification both at the phylogenetic and the antigenic level was evident for the H1N1av lineage and for some of its reassortants. The H1avN2 reassortant R1931/11 displayed conspicuously distinct genetic and antigenic features and easily transmitted from pig-to-pig in an experimental infection. Continuing diverging evolution was also observed in the H1pdmN2 lineage. These viruses carry seven genome segments of the H1N1pdm virus including a hemagglutinin gene which encodes an antigenically markedly altered protein. The zoonotic potential of this lineage remains to be determined. The results highlight the relevance of surveillance and control of porcine influenza virus infections. This is important with respect to health status of swine herds. In addition, a more exhaustive tracing of the formation, transmission and spread of new reassortant influenza A viruses with unknown zoonotic potential is urgently required.