Atopic dermatitis is an inflammatory and chronically relapsing skin disorder that commonly occurs in children; the number of atopic dermatitis patients is increasing. The cause and mechanism of atopic dermatitis have not been defined clearly, although many studies are ongoing. Epidemiological studies suggest that soybean and its isoflavones have immunoregulatory activities. Here, we report that 7,3',4'-trihydroxyisoflavone (7,3',4'-THIF), a major metabolite of daidzin, effectively inhibited lipopolysaccharide (LPS)-induced nitric oxide (NO), tumor necrosis factor (TNF)- α , and interleukin (IL)-6 production in RAW 264.7 cells, and also reduced β -hexosaminidase secretion in RBL-2H3 cells. Moreover, 7,3',4'-THIF significantly reduced scratching time, transepidermal water loss, and mast cell infiltration. It also decreased protease-activated receptor (PAR)-2 and IL-4 expression and increased filaggrin expression in skin lesions of NC/Nga mice. These results suggest that 7,3',4'-THIF improves Dermatophagoides farina body extract-induced atopic dermatitis in NC/Nga mice.