The mammalian target of rapamycin (mTOR) complex 1 (mTORC1) is a nutrient-sensitive protein kinase that is aberrantly activated in many human cancers. Whether dysregulation of mTORC1 signaling in normal tissues increases the risk for cancer, however, is unknown. We focused on hepatocellular carcinoma, which has been linked to environmental factors that affect mTORC1 activity, including diet. Ablation of the gene encoding TSC1 (tuberous sclerosis complex 1), which as part of the TSC1-TSC2 complex is an upstream inhibitor of mTORC1, results in constitutively increased mTORC1 signaling, an effect on this pathway similar to that of obesity. We found that mice with liver-specific knockout of Tsc1 developed sporadic hepatocellular carcinoma with heterogeneous histological and biochemical features. The spontaneous development of hepatocellular carcinoma in this mouse model was preceded by a series of pathological changes that accompany the primary etiologies of this cancer in humans, including liver damage, inflammation, necrosis, and regeneration. Chronic mTORC1 signaling led to unresolved endoplasmic reticulum stress and defects in autophagy, factors that contributed to hepatocyte damage and hepatocellular carcinoma development. Therefore, we conclude that increased activation of mTORC1 can promote carcinogenesis and may thus represent a key molecular link between cancer risk and environmental factors, such as diet.