The B lymphocyte–induced maturation protein 1 (Blimp-1) transcriptional repressor is required for terminal differentiation of B lymphocytes. Here we document a function for Blimp-1 in the T cell lineage. Blimp-1-deficient thymocytes showed decreased survival and Blimp-1-deficient mice had more peripheral effector T cells. Mice lacking Blimp-1 developed severe colitis as early as 6 weeks of age, and Blimp-1-deficient regulatory T cells were defective in blocking the development of colitis. Blimp-1 mRNA expression increased substantially in response to T cell receptor stimulation. Compared with wild-type CD4⁺ T cells, Blimp-1-deficient CD4⁺ T cells proliferated more and produced excess interleukin 2 and interferon-γ but reduced interleukin 10 after T cell receptor stimulation. These results emphasize a crucial function for Blimp-1 in controlling T cell homeostasis and activation.