Interleukin-33 (IL-33), a member of the IL-1 cytokine family, is constitutively expressed in barrier cells such as endothelial cells and fibroblasts. While the expression of IL-33 in regulatory T cells (Tregs) has been previously reported, its clinical significance in pancreatic ductal adenocarcinoma (PDAC) remains unclear. This study aims to investigate the clinical relevance and biological role of IL-33 + Tregs in PDAC.Infiltration of IL-33 + Tregs was assessed by immunohistochemistry in 215 patients from our institute. The correlation between IL-33 + Tregs and clinical characteristics was analyzed. Additionally, the functional status of cytotoxic T cells in relation to IL-33 + Treg infiltration was examined. The impact of IL-33 + Tregs on the tumor microenvironment (TME) was further evaluated both in silico and in vitro .IL-33 + Tregs infiltration was confirmed in PDAC tissues, and its abundance was positively associated with microvascular invasion, perineural invasion, and elevated serum CA19-9 levels. Patients with higher tumor-infiltrating IL-33 + Tregs demonstrated poorer overall survival (OS) and recurrence-free survival (RFS) compared to those with lower infiltration levels. Multivariate analysis confirmed IL-33 + Tregs as an independent prognostic factor for both OS and RFS, with improved survival prediction when combined with tumor differentiation. Subgroup analyses indicated that serum CA19-9 was not a useful risk stratification tool in patients with high IL-33 + Treg infiltration, and these patients showed limited survival benefit from adjuvant chemotherapy. Furthermore, increased IL-33 + Treg infiltration was associated with more pronounced immunosuppressive TME, marked by a reduction in cytotoxic phenotypes and an upregulation of exhausted markers on CD8 + T cells.Our findings identify IL-33 + Tregs as a novel subtype of Tregs, with strong prognostic value for survival risk stratification and therapeutic response prediction in PDAC. IL-33 + Tregs exhibit more pronounced immunosuppressive capabilities, impairing CD8 + T cell function. With further investigation, IL-33 + Tregs may represent a promising immunotherapeutic target for PDAC.Copyright © 2025 The Author(s). Published by Wolters Kluwer Health, Inc.