Enhancer of zeste homolog 2 (EZH2) is a candidate oncogenic driver due to its prevalent overexpression and aberrant repression of tumor suppressor genes in diverse cancers. Therefore, blocking EZH2 enzyme activity may present a valid therapeutic strategy for the treatment of cancers with EZH2 overexpression including breast cancers. Here, we described ZLD1039 a potent, highly selective, and orally bioavailable small molecule inhibitor of EZH2, which inhibited breast tumor growth and metastasis. ZLD1039 considerably inhibited EZH2 methyltransferase activity with nanomolar potency, decreased global histone-3 lysine-27 (H3K27) methylation, and reactivated silenced tumor suppressors connected to increased survival of patients with breast cancer. Comparable to conditional silencing of EZH2, its inhibition by ZLD1039 decreased cell proliferation, cell cycle arrest, and induced apoptosis. Comparably, treatment of xenograft-bearing mice with ZLD1039 led to tumor growth regression and metastasis inhibition. These data confirmed the dependency of breast cancer progression on EZH2 activity and the usefulness of ZLD1039 as a promising treatment for breast cancer.