(R)-ZINC-3573 is a selective agonist probe of the orphan receptor Mas-related G-protein coupled receptor member X2 (MRGPRX2), a G-protein coupled receptor (GPCR) selectively expressed in mast cells and dorsal root and trigeminal ganglia primary sensory neurons of primates. Lack of selective and potent probes has made it difficult to determine MRGPRX2 funtion. (R)-ZINC-3573 was shown to be a potent and selective agonist with an EC50 value of 760 nM for MRGPRX2 and little activity for MRGPRX1 or over 350 other GPCRs and 97 kinases tested. (R)-ZINC-3573 activates endogenous MRGPRX2 expressed in a human mast cell line and induced degranulation and calcium release. (S)-ZINC-3573 is inactive and can be used as a control.