Fucoidan, a polysaccharide extracted from brown seaweeds, reduces tumor cell proliferation. Fucoidan inhibits the growth of breast cancer cells such as 4T1 and MDA-MB-231 and decreases their cell colony formation. Moreover, fucoidan reduces metastatic lung nodules in 4T1 xenograft female Balb/c mice. The molecular network of transforming growth factor β (TGFβ) receptors (TGFRs) plays an important role in the regulation of the epithelial to mesenchymal transition (EMT) in cancer cells. Using 4T1 and MDA-MB-231 cells, we found that fucoidan effectively reverses TGFR-induced EMT morphological changes, upregulates epithelial markers, downregulates mesenchymal markers and decreases the expression of transcriptional repressors Snail, Slug and Twist. Moreover, fucoidan inhibits migration and invasion during the EMT, suggesting the involvement of TGFR-mediated signaling in breast cancer cells. Fucoidan decreases TGFRI and TGFRII proteins and affects downstream signaling molecules, including Smad2/3 phosphorylation and Smad4 expression. In order to elucidate how fucoidan decreases TGFRI and TGFRII proteins in MDA-MB-231 cells, we investigated ubiquitination activity downregulation of TGFRs. It was found that fucoidan enhances proteasome-mediated degradation/ubiquitination of TGFR. This study is the first to identify a novel mechanism for fucoidan antitumor activity, namely regulation of the EMT via modulation of TGFR/Smad-dependent signaling, which leads to an inhibition of breast cancer cell growth in vitro and in vivo. Our current findings indicate that fucoidan is a potential therapeutic agent for breast cancer and acts via an ubiquitin-dependent degradation pathway that affects the TGFR/Smad/Snail, Slug, Twist and EMT axes.