Tumor metastasis is the main cause of cancer-related deaths of patients. Breast cancer is highly malignant with considerable metastatic potential, which urges the necessity for developing novel potential drug candidate to prevent tumor metastasis. Here, we report our finding with Cucurbitacin E (CuE, α-elaterin), a tetracyclic triterpenes compound isolated from Cucurbitaceae. The potency of CuE on breast cancer metastasis inhibition was assessed in vivo and in vitro. In our animal experiments, intraperitoneal administrations of CuE significantly inhibited breast tumor metastasis to the lung without affecting apoptosis or proliferation of inoculated 4T1 and MDA-MB-231 breast cancer cells. Treatment of metastatic breast tumor cells with CuE markedly blocked tumor cell migration and invasion in vitro. Subsequent studies showed that CuE impaired Arp2/3-dependent actin polymerization and suppressed Src/FAK/Rac1/MMP involved pathway. Overall, our data demonstrate that CuE blocks breast cancer metastasis by suppressing tumor cell migration and invasion. We provide first evidence of a novel role for CuE as a potential candidate for treating breast cancer metastasis.