In this study, we investigated the anti-allergic effect of a methanol extract from Polysiphonia marrowii Harvey (PHM) in the immunoglobulin E (IgE)/bovine serum albumin (BSA)-stimulated bone marrow-derived cultured mast cells (BMCMCs) and a passive cutaneous anaphylaxis(PCA) mice model. First, we identified PHM did not show the cytotoxicities at the used all concentrations. In addition, PHM reduced the β-hexosaminidase release known as a biomarker of degranulation in IgE/BSA-stimulated BMCMCs in a dose dependent manner. Also, PHM reduced the expression levels of cytokines such as interleukin (IL)-4, IL-6, IL-10 and tumor necrosis factor (TNF)-α, compared to the only IgE/BSA-stimulated BMCMCs. Furthermore, the treatment of PHM improved the IgE-mediated passive cutaneous anaphylaxis (PCA) reaction known as a type 1 allergic reaction in IgE/BSA-stimulated mice model. Taken together, these results suggest that PHM may be the natural agent with beneficial potentials for the treatment of type I allergic diseases.