Regulatory T cells (Tregs) are indispensable for the control of immune homeostasis and have clinical potential as a cell therapy for treating autoimmunity. Tregs can lose expression of the lineage-defining Foxp3 transcription factor and acquire effector T cell (Teff) characteristics, a process referred to as Treg plasticity. The extent and reversibility of such plasticity during immune responses remain unknown. Here, using a murine genetic fate-mapping system, we show that Treg stability is maintained even during exposure to a complex microbial/antigenic environment. Furthermore, we demonstrate that the observed plasticity of Tregs after adoptive transfer into a lymphopenic environment is a property limited to only a subset of the Treg population, with the nonconverting majority of Tregs being resistant to plasticity upon secondary stability challenge. The unstable Treg fraction is a complex mixture of phenotypically distinct Tregs, enriched for naïve and neuropilin-1-negative Tregs, and includes peripherally induced Tregs and recent thymic emigrant Tregs These results suggest that a "purging" process can be used to purify stable Tregs that are capable of robust fate retention, with potential implications for improving cell transfer therapy.