Selective activation of the estrogen receptor β (ERβ) could be a safe approach to hormone replacement therapy for both women and men, in contrast to the estrogens currently used for women which activate both ERβ and ERα, occasionally causing severe side effects. The selective ERβ agonist AC-131 has shown efficacy in animal models of Parkinson’s disease and neuropathic pain. With the use of AC-131 as template, herein we report the discovery, synthesis, and structure–activity relationship (SAR) study of a new class of dihydrobenzofurans as potent and selective ERβ agonists. The SAR was established by enantioselective synthesis, molecular modeling, and whole-cell-based functional assays. The most potent diastereomer, cis-10-SR, was shown to have an EC50 value of <1 nM, potency 100-fold higher than that of AC-131. Even more interestingly, compound trans-10-SS exhibited 1000-fold ERβ/ERα selectivity while still maintaining good potency (∼10 nM). In addition, trans-10-SS showed only partial agonist activity (30–60 % Eff.) toward ERα at 10 μM. This unprecedented selectivity could be rationalized by molecular modeling. Compound trans-10-SS appears to be the first molecule to take advantage of both conservative amino acid differences found in the α- and β-faces of the binding cavities of ERα and ERβ.