Estrogen receptor-α (ER) is the driving transcription factor in most breast cancers, and its associated proteins can influence drug response, but direct methods for identifying interacting proteins have been limited. We purified endogenous ER using an approach termed RIME (rapid immunoprecipitation mass spectrometry of endogenous proteins) and discovered the interactome under agonist- and antagonist-liganded conditions in breast cancer cells, revealing transcriptional networks in breast cancer. The most estrogen-enriched ER interactor is GREB1, a potential clinical biomarker with no known function. GREB1 is shown to be a chromatin-bound ER coactivator and is essential for ER-mediated transcription, because it stabilizes interactions between ER and additional cofactors. We show a GREB1-ER interaction in three xenograft tumors, and using a directed protein-protein approach, we find GREB1-ER interactions in half of ER+ primary breast cancers. This finding is supported by histological expression of GREB1, which shows that GREB1 is expressed in half of ER+ cancers, and predicts good clinical outcome. These findings reveal an unexpected role for GREB1 as an estrogen-specific ER cofactor that is expressed in drug-sensitive contexts. Display omitted ► A proteomic method identifies protein-protein interaction in primary tumors ► GREB1 is the top estrogen-induced ER-interacting protein ► GREB1 is an essential ER cofactor recruited to chromatin ► GREB1 is an independent prognostic marker Discovering endogenous protein-protein interactions has been limited by sensitivity and the requirement for large quantities of starting material. Carroll and colleagues establish a method called RIME that permits rapid and sensitive purification of associated proteins from limited starting material. They show that the most enriched estrogen receptor (ER)-associated factor in breast cancer cells is GREB1, a factor with no known function. GREB1 is shown to be an essential chromatin-bound regulatory protein, which contributes to ER-mediated gene expression.