Critical to the function of mast cells in immune responses including allergy is their production of lipid mediators, among which only omega-6 (ω-6) arachidonate–derived eicosanoids have been well characterized. Here, by employing comprehensive lipidomics, we identify omega-3 (ω-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 ω-3 epoxides, leading to attenuated mast cell activation and anaphylaxis following FcɛRI cross-linking. Mechanistically, the ω-3 epoxides promote IgE-mediated activation of mast cells by downregulating Srcin1, a Src-inhibitory protein that counteracts FcɛRI signaling, through a pathway involving PPARg. Thus, the PAF-AH2–ω-3 epoxide–Srcin1 axis presents new potential drug targets for allergic diseases.