Critical to the function of mast cells in immune responses including allergy is their production of lipid mediators, among which only omega-6 (v-6) arachidonate-derived eicosanoids have been well characterized. Here, by employing comprehensive lipidomics, we identify omega-3 (omega-3) fatty acid epoxides as new mast cell-derived lipid mediators and show that they are produced by PAF-AH2, an oxidized-phospholipid-selective phospholipase A2. Genetic or pharmacological deletion of PAF-AH2 reduced the steady-state production of omega-3 epoxides, leading to attenuated mast cell activation and anaphylaxis following Fc epsilon R1 cross-linking. Mechanistically, the omega-3 epoxides promote IgE-mediated activation of mast cells by downregulating Srcin1, a Src-inhibitory protein that counteracts Fc. RI signaling, through a pathway involving PPAR gamma. Thus, the PAF-AH2-omega-3 epoxide-Srcin1 axis presents new potential drug targets for allergic diseases.