This disclosure relates to modified viruses, e.g., oncolytic vaccinia viruses, which have been modified to contain an exogenous nucleic acid that expresses a protein that modulates STAT3 activity. It is based, at least in part, on the discovery that vaccinia viruses modified to contain nucleic acid encoding PIAS3 and that express PIAS3 or a fragment thereof can inhibit STAT3 activity and enhance the anti-cancer activity of the vaccinia virus. Accordingly, this disclosure provides for oncolytic vaccinia viruses and methods of using them in the treatment of cancers.