Mas-relate G protein-coupled receptor b2 (Mrgprb2) and its human ortholog MRGPRX2 are specifically expressed in mast cells and widely recognizes cationic ligands such as neuropeptides, bacterial components and some drugs including vancomycin and morphine. These ligands of Mrgprb2/MRGPRX2 involved in (pseudo) allergic reactions, pain and infectious diseases. Previously, we reported that extracellular ATP potentiates antigen-induced mast cell degranulation via P2X4 receptor activation. In this study, we investigated the effect of P2X4 receptor signaling on degranulation induced by Mrgprb2 activation. Stimulation of mouse peritoneal mast cell (PMC) with Mrgprb2 agonist compound 48/80 (C48/80) induced degranulation in a concentration-dependent manner. C48/80-induced degranulation was potentiated by ATP but not by ADP, UTP and UDP. Similar results were obtained with another Mrgprb2 agonists substance P and PAMP-12. The potentiation by ATP was absent in PMC prepared from P2X4 receptor deficient mice. These results suggest that Mrgprb2-mediated mast cell activation is potentiated by the P2X4 receptor signaling.