The number of Foxp3⁺ regulatory T cells (T_reg cells) must be tightly controlled for efficient suppression of autoimmunity with no impairment of normal immune responses. Here we found that the adaptor TRAF3 was intrinsically required for restraining the lineage determination of thymic T_reg cells. T cell–specific deficiency in TRAF3 resulted in a two- to threefold greater frequency of T_reg cells, due to the more efficient transition of precursors of T_reg cells into Foxp3⁺ T_reg cells. TRAF3 dampened interleukin 2 (IL-2) signaling by facilitating recruitment of the tyrosine phosphatase TCPTP to the IL-2 receptor complex, which resulted in dephosphorylation of the signaling molecules Jak1 and Jak3 and negative regulation of signaling via Jak and the transcription factor STAT5. Our results identify a role for TRAF3 as an important negative regulator of signaling via the IL-2 receptor that affects the development of T_reg cells.