Regulatory T cells (T_reg cells) have a pivotal role in the establishment and maintenance of immunological self-tolerance and homeostasis^,. Transcriptional programming of regulatory mechanisms facilitates the functional activation of T_reg cells in the prevention of diverse types of inflammatory responses^,. It remains unclear how T_reg cells orchestrate their homeostasis and interplay with environmental signals. Here we show that liver kinase B1 (LKB1) programs the metabolic and functional fitness of T_reg cells in the control of immune tolerance and homeostasis. Mice with a T_reg-specific deletion of LKB1 developed a fatal inflammatory disease characterized by excessive T_H2-type-dominant responses. LKB1 deficiency disrupted T_reg cell survival and mitochondrial fitness and metabolism, but also induced aberrant expression of immune regulatory molecules including the negative co-receptor PD-1 and the TNF receptor superfamily proteins GITR and OX40. Unexpectedly, LKB1 function in T_reg cells was independent of conventional AMPK signalling or the mTORC1–HIF-1α axis, but contributed to the activation of β-catenin signalling for the control of PD-1 and TNF receptor proteins. Blockade of PD-1 activity reinvigorated the ability of LKB1-deficient T_reg cells to suppress T_H2 responses and the interplay with dendritic cells primed by thymic stromal lymphopoietin. Thus, T_reg cells use LKB1 signalling to coordinate their metabolic and immunological homeostasis and to prevent apoptotic and functional exhaustion, thereby orchestrating the balance between immunity and tolerance.