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Thimerosal has been used as a preservative in a variety of products which may cause contact dermatitis. It is the second most common allergen in positive patch test reactions, though being a clinical irrelevant allergen. It is believed that thimerosal may cause contact dermatitis through delayed-type hypersensitivity response. However, recent studies have demonstrated the association between Mas-related G protein coupled receptor X2 (MRGPRX2) on mast cells and pseudo-allergic reaction. It is possible that thimerosal may cause contact dermatitis via MRGPRX2 mediated mechanism. To investigate the role of MRGPRX2/MrgprB2 in contact dermatitis induced by thimerosal, we developed a novel skin pseudo-allergic reaction mouse model, footpad swelling and extravasation assays in vivo and mast cell degranulation assay in vitro were studied. The results showed that thimerosal induced contact dermatitis in dorsal skin and footpad swelling in wild-type mice, but had no significant effect in MrgprB2knockout mice. Thimerosal-induced dermatitis is characterized by infiltration of inflammatory cells and elevation of serum histamine and inflammatory cytokines, rather than elevation of serum IgE level. Moreover, thimerosal increased the intracellular Ca 2+ concentration in HEK293 cells overexpressing MrgprB2/MRGPRX2. Downregulation of MRGPRX2 resulted in the reduced degranulation of human mast cells from LAD2 cell line. Collectively, we demonstrate that MrgprB2 mediated thimerosal-induced mast cell degranulation and pseudoallergic reaction in mice, thimerosal also activated human mast cell via MRGPRX2, indicating that MRGPRX2 may be a key contributor to human contact dermatitis.