Rationale: Forkhead box P3(+) T regulatory cells (Tregs) are key players in maintaining immune homeostasis. Evidence suggests that Tregs respond to environmental cues to permit or suppress inflammation. In atherosclerosis, Th1-driven inflammation affects Treg homeostasis, but the mechanisms governing this phenomenon are unclear. Objective: Here, we address whether atherosclerosis impacts Treg plasticity and functionality in Apoe(-/-) mice, and what effect Treg plasticity might have on the pathology of atherosclerosis. Methods and Results: We demonstrate that atherosclerosis promotes Treg plasticity, resulting in the reduction of CXCR3(+) Tregs and the accumulation of an intermediate Th1-like interferon (IFN)-gamma(+) CCR5(+) Treg subset (Th1/Tregs) within the aorta. Importantly, Th1/Tregs arise in atherosclerosis from bona fide Tregs, rather than from T-effector cells. We show that Th1/Tregs recovered from atherosclerotic mice are dysfunctional in suppression assays. Using an adoptive transfer system and plasticity-prone Mir146a-/-Tregs, we demonstrate that elevated IFN gamma(+)Mir146a(-/-)Th1/Tregs are unable to adequately reduce atherosclerosis, arterial Th1, or macrophage content within Apoe(-/-) mice, in comparison to Mir146a(+/+) Tregs. Finally, via single-cell RNA-sequencing and real-time-polymerase chain reaction, we show that Th1/Tregs possess a unique transcriptional phenotype characterized by coexpression of Treg and Th1 lineage genes and a downregulation of Treg-related genes, including Ikzf2, Ikzf4, Tigit, Lilrb4, and Il10. In addition, an ingenuity pathway analysis further implicates IFN gamma, IFN alpha, interleukin-2, interleukin-7, CTLA-4 (cytotoxic T-lymphocyte-associated protein 4), T-cell receptor, and Csnk2b-related pathways in regulating Treg plasticity. Conclusions: Atherosclerosis drives Treg plasticity, resulting in the accumulation of dysfunctional IFN gamma(+) Th1/Tregs that may permit further arterial inflammation and atherogenesis.