Diisodecyl phthalate (DIDP) is extensively used as an environmentally friendly plasticizer. However, little is known about the adverse effects and the underlying mechanisms of DIDP exposure on immunological diseases. We aimed to determine the role and mechanisms of DIDP exposure in allergic contact dermatitis-like skin lesions. We show that oral DIDP exposure can aggravate allergic dermatitis in mice. Moreover, an increase of ROS, total serum IgE and IL-4 levels were concomitant with this deterioration. We detected the expression of Thymic stromal lymphopoietin (TSLP) and the activation of STATs and NF-κB signal pathways. The data indicated that DIDP in combination with FITC triggers TSLP production. Our results also suggested that DIDP exacerbated the activation of NF-κB signal pathways, with an enhancement in TSLP expression, which potentiated the activation of STATs and the degranulation of mast cells in the skin, and finally exacerbated allergic dermatitis. The study also suggested that melatonin enhanced the expression of Nrf2, up-regulated the antioxidant genes HO-1 and NQO1, reduced the levels of oxidative stress and TSLP, and alleviated allergic dermatitis. The results demonstrated that DIDP exacerbated allergic dermatitis through oxidative stress and enhanced TSLP production.