Type 2 innate lymphoid cells (ILC2) in lungs produce interleukin (IL)-5 and IL-13 in response to IL-33 and may contribute to the development of allergic diseases such as asthma. However, little is known about negative regulators and effective inhibitors controlling ILC2 function. Here, we show that soluble ST2, a member of the IL-1 receptor family, suppresses the effect of IL-33 on lung ILC2 in vitro. Stimulation with IL-33 to naïve ILC2 induced morphological change and promoted cell proliferation. In addition, IL-33 upregulated expression of cell surface molecules including IL-33 receptor and induced production of IL-5 and IL-13, but not IL-4. Pretreatment with soluble ST2 suppressed IL-33-mediated responses of ILC2. The results suggest that soluble ST2 acts as a decoy receptor for IL-33 and protects ILC2 from IL-33 stimulation.