Breast cancer metastasis is a major cause of cancer-related death in women. However, markers for diagnosisof breast cancer metastasis are rare. Here, we reported that TET1, a tumor suppressor gene, was downregulatedand hypermethylated in highly metastatic breast cancer cell lines. Moreover, silencing of TET1 in breast cancercells increased the migration and spreading of breast cancer cells. In breast cancer clinical samples, TET1expression was reduced in LN metastases compared with primary tissues. Besides, the methylation level ofthe TET1 promoter was increased significantly in LN metastases. Taken together, these findings indicate thatpromoter hypermethylation may contribute to the downregulation of TET1 and could be used as a promisingmarker for diagnosis in patients with breast cancer metastasis.