Infection of cells with vaccinia virus (VACV) results in morphological changes reminiscent of epithelial-mesenchymal transition (EMT), a process that occurs during embryonic morphogenesis, cancer metastasis, and wound healing. The process of EMT may involve several different signalling pathways, including TGF-β, Wnt and Notch pathways. Previous observations in our lab have revealed that elements of the TGF-β/Smad pathway are active following VACV infection. VACV induces a SMAD4-dependent transcriptional response that leads to expression of TGFβ targets such as PAI-1. While cells carrying inducible RNAi for Smad4 are available, no such system exists for studying Smad2 and Smad3 (R-Smads or receptor Smads) in a loss-of-function approach. To this end, targeted siRNA were employed to transiently knockdown levels of either Smad2 or Smad3. We observed that deficiencies in the R-Smad proteins not only severely attenuated VACV infection but also prevented the characteristic morphological changes normally observed following infection. Ultimately, these results support a role for VACV-mediated Smad signalling in eliciting an EMT-like phenotype to promote virus spread. While current trends in EMT-related research are largely within the contexts of either of developmental biology and cancer metastasis, these findings indicate a potentially important, and as-yet under appreciated, role for EMT in pathogenesis.