Opioid receptors (ORs) mediate the actions of endogenous and exogenous opioids for many essential physiological processes including regulation of pain, respiratory drive, mood, and, in the case of κ-opioid receptors (KOR), dysphoria and psychotomimesis. Here we report the crystal structure of the human KOR (hKOR) in complex with the selective antagonist JDTic, arranged in parallel-dimers, at 2.9 angstrom resolution. The structure reveals important features of the ligand binding pocket that contribute to JDTic’s high affinity and subtype-selectivity for hKOR. Modeling of other important KOR-selective ligands, including the morphinan-derived antagonists nor-BNI and GNTI, and the diterpene agonist salvinorin A analog RB-64, reveals both common and distinct features for binding these diverse chemotypes. Analysis of site-directed mutagenesis and ligand structure-activity relationships confirms the interactions observed in the crystal structure, thereby providing a molecular explanation for hKOR subtype-selectivity along with insight essential for the design of hKOR compounds with new pharmacological properties.