Uterine leiomyomas (ULs) are benign uterine tumors, and the most notable pathophysiologic feature of ULs is excessive accumulation of extracellular matrix (ECM). Fucoidan is a polysaccharide extracted from brown seaweeds that has a wide range of pharmacological properties, including anti-fibrotic effects. We aimed to study the effect of fucoidan on the growth of ULs activated by transforming growth factor beta (TGFβ). We used ELT-3 (Eker rat leiomyoma tumor-derived cells) and HUtSMC (human uterine smooth muscle cells) as in vitro models. Cell viability was determined by the MTT assay. Cell colony formation was stained using crystal violet. The side population, cell cycle and apoptosis were analyzed using flow cytometry. Protein expression was assayed by western blot analysis. We also conducted in vivo experiments to confirm the inhibitory effects of fucoidan in nude mouse xenograft models. Tumor tissues were assayed by immunohistochemistry analysis. In our study, fucoidan caused a 50% growth inhibition using a dose of 0.5 mg/ml and decreased the stem cell activity after 48 h. In addition, fucoidan induced sub-G1 cell cycle arrest and apoptosis. Fucoidan down-regulated fibronectin, vimentin, α-SMA and the COL1A1 protein levels in TGFβ3-induced ELT-3 cells. In the cellular mechanism, fucoidan abrogated TGFβ3-induced levels of p-Smad2 and p-ERK1/2, as well as β-catenin translocation into the nucleus. Furthermore, fucoidan suppressed xenograft tumor growth in vivo. Fucoidan displays anti-proliferation and anti-fibrotic effects and exerts protective effects against ULs development.