Foxp3-expressing regulatory T (Treg) cells are essential regulators in the immune system; molecular mechanisms underlying Treg cell expansion and function are still not well understood. SUMOylation is an important post-translational modification characterized by covalent attachment of SUMO moieties to lysine within proteins. UBC9 is the only E2 conjugation enzyme involved in this process and loss of UBC9 completely impairs the SUMOylation pathway. Here we report that selective deletion of Ubc9 within the Treg cell lineage resulted in fatal early-onset autoimmunity as the Foxp3 mutant mice. Ubc9-deficient Treg cells exhibited severe defects in TCR-driven homeostatic proliferation, accompanied by impaired activation and compromised suppressor function. Importantly, TCR-enhanced SUMOylation of IRF4, a critical regulator of Treg cell function downstream of TCR signals, regulates its stability in Treg cells. Our data thus have demonstrated an essential role of SUMOylation in the expansion and function of Treg cells. RNA-seq library was generated using mRNA of CD4+ YFP+ Treg cells sorted from lymph nodes and spleen of Foxp3cre/wtUbc9fl/wt or Foxp3cre/wtUbc9fl/fl mice, each sample contained pooled Treg cells from 5~10 mice.