The role of IL‐33/ST2 pathway in antitumor immunity is unclear. Using 4T1 breast cancer model we demonstrate time‐dependent increase of endogenous IL‐33 at both the mRNA and protein levels in primary tumors and metastatic lungs during cancer progression. Administration of IL‐33 accelerated tumor growth and development of lung and liver metastases, which was associated with increased intratumoral accumulation of CD11b⁺Gr‐1⁺ TGF‐β1⁺ myeloid‐derived suppressor cells (MDSCs) that expressed IL‐13α1R, IL‐13‐producing Lin⁻Sca‐1⁺ST2⁺ innate lymphoid cells (ILCs) and CD4⁺Foxp3⁺ST2⁺IL‐10⁺ Tregs compared to untreated mice. Higher incidence of monocytic vs. granulocytic MDSCs and plasmocytoid vs. conventional dendritic cells (DCs) was present in mammary tumors of IL‐33‐treated mice. Intratumoral NKp46⁺NKG2D⁺ and NKp46⁺FasL⁺ cells were markedly reduced after IL‐33 treatment, while phosphate‐buffered saline‐treated ST2‐deficient mice had increased frequencies of these tumoricidal natural killer (NK) cells compared to untreated wild‐type mice. IL‐33 promoted intratumoral cell proliferation and neovascularization, which was attenuated in the absence of ST2. Tumor‐bearing mice given IL‐33 had increased percentages of splenic MDSCs, Lin⁻Sca‐1⁺ ILCs, IL‐10‐expressing CD11c⁺ DCs and alternatively activated M2 macrophages and higher circulating levels of IL‐10 and IL‐13. A significantly reduced NK cell, but not CD8⁺ T‐cell cytotoxicity in IL‐33‐treated mice was observed and the mammary tumor progression was not affected when CD8⁺ T cells were in vivo depleted. We show a previously unrecognized role for IL‐33 in promoting breast cancer progression through increased intratumoral accumulation of immunosuppressive cells and by diminishing innate antitumor immunity. Therefore, IL‐33 may be considered as an important mediator in the regulation of breast cancer progression.