Estrogen receptor (ER)-positive breast cancer is the most prevalent molecular subtype, and endocrine therapies targeting ER signaling remain a cornerstone of treatment. However, their clinical benefit is often limited by acquired resistance to tamoxifen and the CDK4/6 inhibitor palbociclib. Because ER signaling is frequently retained as a key oncogenic driver even in resistant tumors, targeted ER degradation has emerged as an attractive therapeutic strategy. In this study, we developed ER-ATTECs (estrogen receptor-autophagosome tethering chimeras), heterobifunctional molecules designed to bind ER and LC3, thereby recruiting ER to autophagosomes for selective autophagy-mediated degradation. ER-ATTEC induced ERα degradation and exerted anti-cancer activity in MCF7 cells. In vivo, ER-ATTEC suppressed tumor growth in MCF7 xenograft models. Importantly, ER-ATTEC exhibited greater anti-cancer activity than the ER-targeting PROTAC ARV-471 in tamoxifen-resistant MCF7-Tam1 and palbociclib-resistant T47D/PalR cells, where ER degradation was also confirmed. These findings suggest that enhanced autophagic flux in resistant cells may potentiate ATTEC activity relative to proteasome-dependent PROTACs. Collectively, our results demonstrate that ER-ATTEC induces autophagy-mediated ER degradation and retains potent anti-cancer activity in drug-resistant ER-positive breast cancer, supporting ATTEC as a promising strategy to overcome endocrine resistance.