Adaptive immunity has a major impact on atherosclerosis, with pro- and anti-atherosclerotic effects exerted by different subpopulations of T cells. Transforming growth factor–β (TGF-β) may promote development either of anti-atherosclerotic regulatory T cells or of T helper 17 (T_H17) cells, depending on factors in the local milieu. We have addressed the effect on atherosclerosis of enhanced TGF-β signaling in T cells. Bone marrow from mice with a T cell–specific deletion of Smad7, a potent inhibitor of TGF-β signaling, was transplanted into hypercholesterolemic Ldlr^−/− mice. Smad7-deficient mice had significantly larger atherosclerotic lesions that contained large collagen-rich caps, consistent with a more stable phenotype. The inflammatory cytokine interleukin-6 (IL-6) was expressed in the atherosclerotic aorta, and increased mRNA for IL-17A and the T_H17-specific transcription factor RORγt were detected in draining lymph nodes. Treating Smad7-deficient chimeras with neutralizing IL-17A antibodies reversed stable cap formation. IL-17A stimulated collagen production by human vascular smooth muscle cells, and RORγt mRNA correlated positively with collagen type I and α-smooth muscle actin mRNA in a biobank of human atherosclerotic plaques. These data link IL-17A to induction of a stable plaque phenotype, could lead to new plaque-stabilizing therapies, and should prompt an evaluation of cardiovascular events in patients treated with IL-17 receptor blockade.