The homeostasis of naive T cells is essential for protective immunity against infection, but the cell-intrinsic molecular mechanisms that control naïve T-cell homeostasis are poorly understood. Genetic ablation in lower organisms has revealed a critical role for Vps34, an evolutionary conserved class III phosphoinositide-3 kinase (PI3K), in regulating endocytosis and autophagy; however, the physiological function of Vps34 in the immune system, especially in T cells, is unclear. Here we report that Vps34 is required for the maintenance of naïve T cells, acting in a cell-intrinsic manner. T-cell-specific deletion of the gene encoding Vps34 resulted in reduced stability of Vps15 and Beclin-1, components of the class III PI3K complex, and impaired autophagy in T cells. Vps34 was dispensable for T-cell development but important for the survival of naïve T cells. Vps34-deficient T cells showed increased mitochondrial mass and accumulation of reactive oxygen species, consistent with deficient removal of damaged mitochondria. Thus, Vps34-dependent canonical autophagy plays a critical role in maintaining T-cell homeostasis by promoting T-cell survival through quality control of mitochondria.