Garlic (Allium sativum) has been used as a food as well as a component of traditional medicine. Aged black garlic (ABG) is known to have various bioactivities. However, the effect of ABG on allergic response is almost unknown. In the present study, we investigated whether ABG can inhibit immunoglobulin E-mediated allergic response in RBL-2H3 cells as well as in vivo passive cutaneous anaphylaxis (PCA). In in vitro tests, ethyl acetate extract (EBG) of ABG significantly inhibited the release of β-hexosaminidase (IC₅₀, 1.53 mg/mL) and TNF-α (IC₅₀, 0.98 mg/mL). Moreover, BG10, an active fraction of EBG, dramatically suppressed the release of β-hexosaminidase (IC₅₀, 53.60 μg/mL) and TNF-α (IC₅₀, 27.80 μg/mL). In addition, BG10 completely blocked the formation of prostaglandin E₂ and leukotriene B₄ at ≥25 μg/mL. When the effect of BG10 on FcɛRI receptor cascade was investigated, BG10 significantly inhibited the phosphorylation of Syk, but not Lyn. Furthermore, BG10 dose dependently decreased the phosphorylation of cytosolic phospholipase A₂ (cPLA₂) and 5-lipoxygenase (5-LO) as well as the expression of cyclooxygenase-2 (COX-2). Consistent with what has been mentioned earlier, BG10 also significantly inhibited the PCA reaction in mice. In conclusion, these results indicate that ABG suppresses the allergic response, and the mechanism for its anti-allergic action may involve suppressions of Syk, cPLA₂, 5-LO, and COX-2. The anti-allergic actions of ABG, EBG, or BG10 suggest that they may be useful as functional foods for allergic diseases.