Agents that inhibit estrogen production, such as aromatase inhibitors or those that directly block estrogen receptor (ER) activity, such as selective estrogen receptor modulators and selective estrogen receptor degraders, are routinely used in the treatment of ER-positive breast cancers. However, although initial treatment with these agents is often successful, many women eventually relapse with drug-resistant breast cancers. To overcome some of the challenges associated with current endocrine therapies and to combat the development of resistance, there is a need for more durable and more effective ER-targeted therapies. Here we describe and characterize a novel, orally bioavailable small-molecule selective estrogen receptor degrader, RAD1901, and evaluate its therapeutic potential for the treatment of breast cancer. RAD1901 selectively binds to and degrades the ER and is a potent antagonist of ER-positive breast cancer cell proliferation. Importantly, RAD1901 produced a robust and profound inhibition of tumor growth in MCF-7 xenograft models. In an intracranial MCF-7 model, RAD1901-treated animals survived longer than those treated with either control or fulvestrant, suggesting the potential benefit of RAD1901 in the treatment of ER-positive breast cancer that has metastasized to the brain. Finally, RAD1901 preserved ovariectomy-induced bone loss and prevented the uterotropic effects of E2, suggesting that it may act selectively as an agonist in bone but as an antagonist in breast and uterine tissues. RAD1901 is currently under clinical study in postmenopausal women with ER-positive advanced breast cancer.