The transcription factor B lymphocyte-induced maturation protein-1 (Blimp-1) plays important roles in embryonic development and immunity. Blimp-1 is required for the differentiation of plasma cells, and mice with T cell-specific deletion of Blimp-1 (Blimp1CKO mice) develop a fatal inflammatory response in the colon. Previous work demonstrated that lack of Blimp-1 in CD4+ and CD8+ T cells leads to intrinsic functional defects, but little is known about the functional role of Blimp-1 in regulating differentiation of Th cells in vivo and their contribution to the chronic intestinal inflammation observed in the Blimp1CKO mice. In this study, we show that Blimp-1 is required to restrain the production of the inflammatory cytokine IL-17 by Th cells in vivo. Blimp1CKO mice have greater numbers of IL-17-producing TCRβ+CD4+cells in lymphoid organs and in the intestinal mucosa. The increase in IL-17-producing cells was not restored to normal levels in wild-type and Blimp-1CKO-mixed bone marrow chimeric mice, suggesting an intrinsic role for Blimp-1 in constraining the production of IL-17 in vivo. The observation that Blimp-1-deficient CD4+ T cells are more prone to differentiate into IL-17+/IFN-γ+ cells and cause severe colitis when transferred to Rag1- deficient mice provides further evidence that Blimp-1 represses IL-17 production. Analysis of Blimp-1 expression at the single cell level during Th differentiation reveals that Blimp-1 expression is induced in Th1 and Th2 but repressed by TGF-β in Th17 cells. Collectively, the results described here establish a new role for Blimp-1 in regulating IL-17 production in vivo. Copyright © 2012 by The American Association of Immunologists, Inc.