G protein-coupled receptor 3 (GPR3) is highly expressed in various neurons and has a unique ability to constitutively activate the Gαs protein without the addition of ligands. However, the expression and physiological roles of GPR3 in nonneuronal cells have not yet been elucidated. This study examined the possible role of GPR3 in T lymphocytes and mast cells. In T lymphocytes, GPR3 was induced as early as 1 to 2 h after T-cell stimulation, and its duration was differently regulated by the signal input. Sustained GPR3 expression by T-cell stimulation upregulated NR4A2, subsequently enhancing NR4A transcriptional activity in Jurkat cells. In mouse primary T lymphocytes, GPR3 had a suppressive role in spontaneous effector T-cell activation. In contrast, GPR3 was also induced as early as 1 to 2 h in mouse bone marrow-derived mast cells (BMMC) by IgE or ATP stimulation. The degranulation of BMMCs by IgE or ATP was significantly inhibited by elevated intracellular cAMP levels. BMMCs from GPR3 knockout mice showed significantly increased degranulation in response to various degranulation stimuli. Moreover, tumor necrosis factor-α and interleukin-6 production significantly increased 24 h after ATP stimulation in GPR3 knockout BMMCs compared to wild-type BMMCs. In conclusion, GPR3 was immediately early induced in T lymphocytes and mast cells and may play a role in the immunosuppressive functions in these cells.