The cytokines of the transforming growth factor–β (TGF-β) family promote the growth and differentiation of multiple tissues, but the role of only the founding member, TGF-β, in regulating the immune responses has been extensively studied. TGF-β is critical to prevent the spontaneous activation of self-reactive T cells and sustain immune homeostasis. In contrast, in the presence of proinflammatory cytokines, TGF-β promotes the differentiation of effector T helper 17 (T_H17) cells. Abrogating TGF-β receptor signaling prevents the development of interleukin-17 (IL-17)–secreting cells and protects mice from T_H17 cell–mediated autoimmunity. We found that the receptor of another member of TGF-β family, bone morphogenetic protein receptor 1α (BMPR1α), regulates T helper cell activation. We found that the differentiation of T_H17 cells from naive CD4⁺ T cells was inhibited in the presence of BMPs. Abrogation of BMPR1α signaling during CD4⁺ T cell activation induced a developmental program that led to the generation of inflammatory effector cells expressing large amounts of IL-17, IFN-γ, and TNF family cytokines and transcription factors defining the T_H17 cell lineage. We found that TGF-β and BMPs cooperated to establish effector cell functions and the cytokine profile of activated CD4⁺ T cells. Together, our data provide insight into the immunoregulatory function of BMPs.