The orphan G protein-coupled receptor (GPCR) GPR84 is expressed on immune cells mediating proinflammatory and immunostimulatory effects. In this study, we prepared the fully efficacious, nonbiased GPR84 agonist 6-hexylamino-2,4(1H,3H)-pyrimidinedione (6) in tritium-labeled form ([3H]PSB-1584) by hydrogenation of a hexenyl-substituted precursor with tritium gas. The radioligand was characterized by kinetic, saturation, and competition assays using membranes of Chinese hamster ovary cells recombinantly expressing the human GPR84. [3H]6 reversibly labeled the receptor with high affinity (KD 2.08 nM). Structurally diverse orthosteric and allosteric ligands, including newly designed and synthesized compounds, were studied in competition binding assays. A homology model of GPR84 was generated to perform docking studies rationalizing the experimental data. The radioligand was additionally used for labeling GPR84 in native cells and tissues. [3H]6 constitutes the first GPR84 agonist radioligand representing a powerful tool for this poorly investigated GPCR, which has potential as a future drug target.