An isoform of the 14-3-3 protein family, 14-3-3θ has been linked with tumor cell proliferation and apoptosis. However, the role of 14-3-3θ in the progression of breast cancer remains unknown. Here, we report that 14-3-3θ plays a critical role in breast cancer metastasis and prognosis. The expression of 14-3-3θ was markedly higher in breast cancer tissues compared to adjacent normal tissues. A hospital-based study cohort of 216 breast cancer patients was evaluated in this study. The level of 14-3-3θ expression was determined and correlated based upon tumor clinicopathological features, disease-free survival, and overall survival. We found that overexpression of 14-3-3θ was correlated with advanced TNM stage (P<0.05), lymph node metastasis (P<0.05), and ER negative status (P<0.05). Breast cancer patients with high 14-3-3θ expression had a shorter overall survival and a higher rate of recurrence than those with low 14-3-3θ expression. Additionally, knockdown of 14-3-3θ expression in breast cancer cells inhibited metastasis in vitro. Similarly, an in vivo assay showed that 14-3-3θ knockdown dramatically suppressed the growth of breast cancer xenografts and inhibited tumor cell metastasis in a lung metastasis model. Thus, this study provided the first evidence that 14-3-3θ is a novel tumor suppressor and may serve as a candidate prognostic biomarker and target for new therapies in metastatic breast cancer.