Mast cells are key players in mediating and amplifying allergic and inflammatory reactions. Previously, we identified the G-protein, Gi3, as the cellular target of receptor mimetic basic secretagogues that activate mast cell independently of IgE. In this study, we demonstrate that Gi3 is the cellular target of the adenosine A3 receptor (A3R), a G-protein coupled receptor involved in inflammation and the pathophysiology of asthma. By using a cell permeable peptide comprising the C-terminal end of Gαi3 fused to an importation sequence (ALL1) as a selective inhibitor of Gi3 signaling, we show that by coupling to Gi3, the A3R stimulates multiple signaling pathways in human mast cells, leading to upregulation of cytokines, chemokines, and growth factors. We further show that after contact with activated T cell membranes, endogenous adenosine binds to and activates the A3R, resulting in Gi3-mediated signaling. Specifically, the majority of ERK1/2 signaling initiated by contact with activated T cell membranes, is mediated by Gi3, giving rise to ALL1-inhibitable cellular responses. These results unveil the physiological G-protein coupled receptor that couples to Gi3 and establish the important role played by this G-protein in inflammatory conditions that involve adenosine-activated mast cells.