Significance G protein-coupled receptors are considered to function principally at the cell surface. We present evidence that the δ-opioid receptor (DOPr) signals from endosomes to cause a sustained inhibition of pain. Opioids from the inflamed human and mouse colon, along with selective agonists that evoked DOPr internalization, inhibited the excitability of nociceptors by a mechanism requiring DOPr endocytosis. DOPr in endosomes generated a subset of signals in subcellular compartments that inhibited neuronal excitability. A DOPr agonist that was encapsulated into nanoparticles designed to selectively activate DOPr in endosomes of nociceptors caused a long-lasting inhibition of neuronal excitability and pain. Our results support the hypothesis that endosomal signaling of DOPr is an endogenous mechanism and therapeutic target for relief from inflammatory pain.