Halofuginone (HF) is extracted from Dichroa febrifuga, a plant used in traditional medicine. We report that the HF extract inhibits the growth of breast cancer cells and induces the generation of reactive oxygen species (ROS) and apoptosis, an important feature of potential anticancer agents. In addition, HF significantly reduces the migration and invasion of MCF-7 and MDA-MB-231 human breast cancer cells after 12-O-tetraecanoylphorbol-13-acetate (TPA) stimulation. As matrix metalloproteinase-9 plays a critical role in tumor metastasis, we analyzed its expression with the HF extract treatment. Western blot analysis and gelatin zymography showed that HF suppresses MMP-9 expression and activity concentration-dependently. HF also decreases the nuclear protein levels of nuclear factor kappa B (NF-κB) and c-fos (AP-1), critical transcription factors regulating MMP-9 expression through binding the MMP-9 promoter region. Luciferase assays showed that HF decreases TPA-induced MMP-9 promoter binding activities of NF-κB and AP-1. Taken together, these are the first results indicating that halofuginone may represent a promising new agent for breast cancer chemotherapy.