Estrogen receptor α (ERα) is a ligand-activated transcription factor that regulates expression of estrogen-responsive genes. Upon binding of the ligand-occupied ERα to estrogen response elements (EREs) in DNA, the receptor interacts with a variety of coregulatory proteins to modulate transcription of target genes. We have isolated and identified a number of proteins associated with the DNA-bound ERα. One of these proteins, Rho guanosine diphosphate (GDP) dissociation inhibitor α (RhoGDIα), is a negative regulator of the Rho family of GTP-binding proteins. In this study, we demonstrate that endogenously expressed RhoGDIα is present in the nucleus as well as the cytoplasm of MCF-7 breast cancer cells, and that RhoGDIα binds directly to ERα, alters the ERα–ERE interaction, and influences the ability of ERα to regulate transcription of a heterologous estrogen-responsive reporter plasmid in transient transfection assays as well as endogenous, estrogen-responsive genes in MCF-7 cells. Our studies suggest that, in addition to the activity of RhoGDIα in the cytoplasm, it also influences ERα signaling in the nucleus.