Classically, G-protein-coupled receptors (GPCRs) are thought to activate G protein from the plasma membrane and are subsequently desensitized by β-arrestin (β-arr). However, some GPCRs continue to signal through G protein from internalized compartments, mediated by a GPCR-G protein-β-arr 'megaplex'. Nevertheless, the molecular architecture of the megaplex remains unknown. Here, we present its cryo-electron microscopy structure, which shows simultaneous engagement of human G protein and bovine β-arr to the core and phosphorylated tail, respectively, of a single active human chimeric β-adrenergic receptor with the C-terminal tail of the arginine vasopressin type 2 receptor (βVR). All three components adopt their canonical active conformations, suggesting that a single megaplex GPCR is capable of simultaneously activating G protein and β-arr. Our findings provide a structural basis for GPCR-mediated sustained internalized G protein signaling.