Steroid hormone receptors (SHRs) are ligand-activated transcription factors that regulate gene expression. SHRs also mediate rapid, nongenomic cellular activation by steroids. In vascular endothelial cells, the SHR for estrogen, estrogen receptor (ER) α, is targeted by unknown mechanisms to a functional signaling module in membrane caveolae that enables estrogen to rapidly activate the mitogen-activated protein kinase and phosphatidylinositol 3–Akt kinase pathways, and endothelial NO synthase (eNOS). Here we identify the 110-kDa caveolin-binding protein striatin as the molecular anchor that localizes ERα to the membrane and organizes the ERα–eNOS membrane signaling complex. Striatin directly binds to amino acids 183–253 of ERα, targets ERα to the cell membrane, and serves as a scaffold for the formation of an ERα–Gαi complex. Disruption of complex formation between ERα and striatin blocks estrogen-induced rapid activation mitogen-activated protein kinase, Akt kinase, and eNOS, but has no effect on ER-dependent regulation of an estrogen response element-driven reporter plasmid. These findings identify striatin as a molecular scaffold required for rapid, nongenomic estrogen-mediated activation of downstream signaling pathways. Furthermore, by demonstrating independent regulation of nongenomic vs. genomic ER-dependent signaling, these findings provide conceptual support for the potential development of “pathway-specific” selective ER modulators.