The increasing occurrences of allergic disorders may be attributed to exposure to environmental factors that contribute to the pathogenesis of allergy. The health benefits of green tea have been widely reported but are largely unsubstantiated. Theanine is the major amino acid present in green tea. In this study, we investigated the role of theanine in both IgE- and non- IgE-induced allergic response. Theanine inhibited compound 48/80-induced systemic anaphylactic shock and ear swelling responses. IgE-mediated passive cutaneous anaphylaxis was inhibited by the oral administration or pharmaceutical acupuncture of theanine. Histamine release from mast cells was decreased with the treatment of theanine. Theanine also repressed phorbol 12-myristate 13-acetate and calcium ionophore A23187-induced TNF-α, IL-1β, IL-6, and IL-8 secretion by suppressing NF-κB activation. Furthermore, theanine suppressed the activation of caspase-1 and the expression of receptor interacting protein-2. The current study demonstrates for the first time that theanine might possess mast cell-stabilizing capabilities.