The R,R enantiomer of 5,11-cis-diethyl-5,6,11,12-tetrahydrochrysene-2,8-diol (THC) exerts opposite effects on the transcriptional activity of the two estrogen receptor (ER) subtypes, ERα and ERβ. THC acts as an ERα agonist and as an ERβ antagonist. We have determined the crystal structures of the ERα ligand binding domain (LBD) bound to both THC and a fragment of the transcriptional coactivator GRIP1, and the ERβ LBD bound to THC. THC stabilizes a conformation of the ERα LBD that permits coactivator association and a conformation of the ERβ LBD that prevents coactivator association. A comparison of the two structures, taken together with functional data, reveals that THC does not act on ERβ through the same mechanisms used by other known ER antagonists. Instead, THC antagonizes ERβ through a novel mechanism we term 'passive antagonism'.