We identified CD8+ CD122+ regulatory T cells (Tregs) and demonstrated their importance in the maintenance of immune homeostasis and in the recovery from experimental autoimmune encephalomyelitis. In this paper, we show that CD8+ CD122+ Tregs effectively prevent and cure colitis in a mouse model. In our experiments, colitis was induced in lymphocyte-deficient RAG-2−/− mice by transferring CD4+ CD45RB high cells that were excluded with CD4+ Tregs. Cotransfer of CD8+ CD122+ cells clearly suppressed the development of colitis, and this suppressive effect was similar to that of CD4+ CD45RB low cells that were mostly CD4+ Tregs. CD8+ CD122+ cells obtained from IL-10−/− mice were unable to suppress colitis, indicating that IL-10 is an important effect-transmitting factor in the suppression of colitis. CD8+ CD122+ cells showed a suppressive effect when they were transferred 4 wk after CD4+ CD45RB high cells, indicating the therapeutic potential of CD8+ CD122+ cells. A mixture of CD8+ CD122+ cells and CD4+ CD45RB low cells was far more effective than single Tregs, indicating the synergistic effect of these Tregs. These overall findings demonstrate the potential role of CD8+ Tregs, and possibly together with CD4+ Tregs, in the medical care of inflammatory bowel disease patients.