Atopic dermatitis (AD) is a chronic inflammation associated with skin hypersensitivity caused by environmental factors. The objent of this study was to assess the hot water extracts of Sargassum horneri (SHHWE) on AD. AD was induced by spreading 2,4-dinitrochlorobenzene (DNCB) on the BALB/c mice. The efficacy of SHHWE was tested by observing the immunoglobulin E (IgE), cytokine, skin clinical severity score and cytokine secretions in concanavalin A (Con A)-stimulated splenocytes. The levels of interleukine (IL)-4, IL-5 and IgE, the pro-inflammatory cytokines that are closely related, were notably suppressed in a does-dependent manner by SHHWE, whereas the level of interferon γ (IFN-γ), the atopy-related Th1 cytokine inhibiting the production of Th2 cytokines, was increased. Therefore, these results show that SHHWE has a potent anti-inhibitory effect on AD and is highly valuable for cosmetic development.