Opioid receptor delta 1 (OPRD1), also known as the δ-opioid receptor (DOR), is a class A G protein-coupled receptor (GPCR) that mediates the biological effects of endogenous opioid peptides such as enkephalins. OPRD1 primarily couples to Gi/Go proteins, leading to inhibition of adenylyl cyclase, reduced intracellular cAMP production, modulation of ion channel activity, and suppression of neurotransmitter release. It is widely expressed in the central and peripheral nervous systems, including the cortex, striatum, hippocampus, spinal cord, and sensory neurons, where it regulates analgesia, mood, reward, emotional processing, and neuroprotection. Compared with other opioid receptors, OPRD1 has attracted particular interest as a target for pain management with potentially reduced respiratory depression and abuse liability, although its pharmacology is complex and context dependent. Dysregulated OPRD1 signaling has been implicated in chronic pain, depression, anxiety, addiction, epilepsy, and neuroinflammatory disorders.