IL-11 expressed by endometrial stromal cells is crucial for normal pregnancy. IL-11 receptor α (IL-11Rα) null mice are infertile due to abnormal development of the placenta. In these mice, the mesometrial decidual tissue, which is the site of trophoblast invasion, thins and disappears at mid-pregnancy. Degeneration of the decidua is accompanied by uncontrolled trophoblast invasion. In this report, we show, using IL-11Rα null mice, that a defect in IL-11 signaling in the decidua leads to severe down-regulation of α₂-macroglobulin (α₂-MG), a metalloproteinase inhibitor crucial for limiting trophoblast invasion. We also present evidence, using uterine stromal cells that decidualize in culture, that IL-11 robustly stimulates the endogenous α₂-MG expression and enhances α₂-MG promoter activity. Serial 5′ deletion and internal deletion of the promoter reveal two important signal transducer and activator of transcription (Stat) binding sites. Mutation of either one of these motifs decreases IL-11 stimulation, whereas double mutation prevents IL-11 action. We also found that IL-11 activates Janus kinase 2 (Jak2) and induces rapid phosphorylation, nuclear translocation, and promoter binding activity of Stat3 in decidual cells, whereas Jak1, Tyk2, and Stat5 activities are not affected. In addition, Jak2 inhibitor totally prevents α₂-MG expression in decidual cells. Taken together, results of this investigation provide, at least in part, an explanation for the overinvasiveness of the trophoblast in IL-11Rα null mice and reveal, for the first time, that IL-11 signals through the Jak2/Stat3 pathway in decidual cells to stimulate the expression of α₂-MG, a protease inhibitor essential for normal placentation in pregnancy.