As an AEG-1/MTDH/LYRIC-binding protein, Staphylococcal nuclease domain-containing 1 (SND1) is upregulated in numerous human cancers where it has been assigned multiple functional roles. In this study, we discovered that SND1 was upregulated in breast cancer tissues, particularly the tissues from patients with distant metastases. The underlying molecular mechanisms demonstrated a novel role of SND1 in regulating the activity of transforming growth factor β1 (TGFβ1) signaling pathway, which promotes metastasis in breast cancer. We illustrated that SND1 physically associated with and recruited the histone acetylase GCN5 to the promoter regions of Smad2/3/4, and consequently enhanced the gene transcriptional activation of Smad2/3/4, which are essential downstream regulators in the TGFβ1 pathway. An electrophoretic mobility shift assay experiment further verified that SND1 could recognize the conserved domains (motifs 1 and 2) in the promoter regions of the Smad genes. Glutathione Stransferase (GST) pulldown assays indicated that the tudor domain of SND1 was responsible for the recruitment of GCN5, which increased histone H3K9 acetylation. Consistent with these results, a loss-of-function of SND1 reduced the protein level of Smads and the phosphorylation of R-Smads, thereby attenuating the R-Smad/Co-Smad depended transcription and, as a result, inhibited TGFβ signaling activation.