Recent evidence indicates that the transactivation of estrogen receptor α (ERα) requires estrogen‐dependent receptor ubiquitination and degradation. Here we show that estrogen‐unbound (unliganded) ERα is also ubiquitinated and degraded through a ubiquitin–proteasome pathway. To investigate this ubiquitin–proteasome pathway, we purified the ubiquitin ligase complex for unliganded ERα and identified a protein complex containing the carboxyl terminus of Hsc70‐interacting protein (CHIP). CHIP preferentially bound to misfolded ERα and ubiquitinated it to induce degradation. Ligand binding to the receptor induced the dissociation of CHIP from ERα. In CHIP−/− cells, the degradation of unliganded ERα was abrogated; however, estrogen‐induced degradation was observed to the same extent as in CHIP+/+ cells. Our findings suggest that ERα is regulated by two independent ubiquitin–proteasome pathways, which are switched by ligand binding to ERα. One pathway is necessary for the transactivation of the receptor and the other is involved in the quality control of the receptor.