The roots of Rumecis Radix have been used for the treatment of heat phlegm, jaundice, constipation, scabies and uterine hemorrhage. The aim of this study was to confirm whether Rumecis Radix water extract (RJWE) has a preventive effect on the development of atopic dermatitis (AD) in 2,4-dinitrochlorobenzene (DNCB)-applied BALB/c mice. Oral administration (12.5 mg/kg, 25 mg/kg) and topical application (0.5 mg/mouse, 1.0 mg/mouse) of RJWE decreased the development of AD-like skin lesions, ear swelling, spleen weight and total serum IgE. RJWE significantly also inhibited the infiltration of mast cells in the dorsal skin. Furthermore, the release of histamine from rat peritoneal mast cells (RPMCs) was suppressed significantly. These results suggest that the inhibitory effect of RJWE on AD might be associated with mast cells.