T-cell anergy is a state of T cells that is hyporesponsive to stimulation via the T-cell receptor and costimulatory molecules and is thought to be important for self-tolerance. How T-cell anergy is regulated is still poorly understood. We report here that tuberous sclerosis (TSC)1 is critical for T-cell anergy. Deficiency of TSC1 resulted in enhanced T-cell proliferation and cytokine production in the absence of cluster of differentiation (CD)28-mediated costimulation, accompanied by enhanced T-cell metabolism. Resistance of TSC1-deficient T cells to anergy is correlated with increased signaling through the mammalian target of rapamycin complex (mTORC)1 and can be reverted by treatment of these cells with mTORC1 inhibitor rapamycin. Expression of the inducible costimulator (ICOS) is increased in TSC1-deficient T cells, which can be inhibited by rapamycin. Simultaneous blockade of both CD28 and ICOS costimulation partially restored sensitivity of TSC1-deficient T cells to anergy induction. Together, our data indicate that TSC1 is crucial for T-cell anergy by inhibiting mTORC1 signaling through both ICOS-dependent and -independent mechanisms.