Regulatory T cells (T reg) are critically involved in maintaining immunological tolerance, but this potent suppression must be quenched to allow the generation of adaptive immune responses. Here we report that type 1 sphingosine-1-phosphate (S1P) receptor (S1P 1) delivers an intrinsic negative signal to restrain thymic generation, peripheral maintenance and suppressive activity of T reg cells. Combining loss- and gain-of-function genetic approaches, we found that S1P 1 blocked the differentiation of thymic T reg precursors and function of mature Treg cells, and affected T reg-mediated immune tolerance. S1P 1 induced the selective activation of the Akt-mTOR pathway to impede T reg development and function. Dynamic regulation of S1P 1 contributed to lymphocyte priming and immune homeostasis. Thus, by antagonizing T reg-mediated immune suppression, the lipid-activated S1P 1-Akt-mTOR pathway orchestrates adaptive immune responses.