We have provided a protocol for establishing an atopic dermatitis (AD) in vitro model, and evaluated the effects of Costaria costata (CC) extracts on AD in an in vitro model using keratinocytes and splenocytes from AD-induced mice and mast cells. HaCaT cells were each treated with 200 μg/mL of CC water extract (CCW), CC 10% ethanol extract (CCE10%), and CC 70% ethanol extract (CCE70%), immediately followed by stimulation with 20 ng/mL tumor necrosis factor (TNF)-α, and 20 ng/mL interferon (IFN)-γ for inflammation. The splenocytes from AD-induced mice were each treated with 200 μg/mL of CCW, CCE10%, and CCE70%, followed by stimulation with 5 μg/mL ConA or lipopolysaccharide (LPS), to induce T cell or B-cell activation, and 5 μg/mL LPS and 50 ng/mL interleukin-4, to induce immunoglobulin (Ig) E production. We investigated the effects of CCW, CCE10%, and CCE70% on the production of histamine in PMA, and A23187-stimulated MC/9 cells. We found that treatments with CC extracts decreased the production of proinflammatory cytokines in TNF-α and IFN-γ-stimulated HaCaT cells, and the suppression of the imbalance of Th1/Th2 cytokines and IgE production on primary splenocytes. In addition, CC extracts resulted in a decrease in histamine release in the PMA and A23187-simulated MC/9 cells. According to our present results, we can conclude that CC extracts may be effective for the treatment of other allergy diseases, and AD, via the attenuation of allergic reactions.