Abstract Metastatic disease is the major cause of breast cancer–related death and despite many advances, current therapies are rarely curative. Tumor cell migration and invasion require actin cytoskeletal reorganization to endow cells with capacity to disseminate and initiate the formation of secondary tumors. However, it is still unclear how these migratory cells colonize distant tissues to form macrometastases. The E6-associated protein, E6AP, acts both as an E3 ubiquitin-protein ligase and as a coactivator of steroid hormone receptors. We report that E6AP suppresses breast cancer invasiveness, colonization, and metastasis in mice, and in breast cancer patients, loss of E6AP associates with poor prognosis, particularly for basal breast cancer. E6AP regulates actin cytoskeletal remodeling via regulation of Rho GTPases, acting as a negative regulator of ECT2, a GEF required for activation of Rho GTPases. E6AP promotes ubiquitination and proteasomal degradation of ECT2 for which high expression predicts poor prognosis in breast cancer patients. We conclude that E6AP suppresses breast cancer metastasis by regulating actin cytoskeleton remodeling through the control of ECT2 and Rho GTPase activity. These findings establish E6AP as a novel suppressor of metastasis and provide a compelling rationale for inhibition of ECT2 as a therapeutic approach for patients with metastatic breast cancer. Cancer Res; 76(14); 4236–48. ©2016 AACR.