CD4 Th cells are organizers of the immune response, directing other immune cells to initiate and maintain effective humoral and cellular immunity. CD4 T cells differentiate into distinct Th effector or regulatory subsets in response to signals delivered to them during the course of infection. Ikaros is a transcription factor that is expressed in blood cells from the level of the hematopoietic stem cell. It is required for normal thymic T cell development and serves as a tumor suppressor, as lack of Ikaros in developing lymphoid cells results in leukemia. To study the role of Ikaros in CD4 T cell differentiation and function, an Ikaros conditional knockout mouse was developed such that Ikaros expression was deleted specifically in mature T cells, thus avoiding defects observed in germline Ikaros mutant mice. Using this model system, we have shown that in the absence of Ikaros, CD4 T cells are able to attain Th1, Th2, and Th17, but not inducible regulatory T, cell fates. However, they show enhanced expression of a cohort of proinflammatory cytokines, resulting in differentiation of Th17 cells with a phenotype that has been associated with autoimmunity and pathological inflammation. In addition, we define Ikaros as a repressor of the gene program associated with the response to type I IFNs, another key pathway whose deregulation is linked to autoimmunity. Taken together, these data definitively define Ikaros as a critical regulator at the center of the inflammatory response in T cells and highlight a potential role in suppressing autoimmunity.