Glycogen and lipid are major storage forms of energy that are tightly regulated by hormones and metabolic signals. Here, we demonstrate that feeding mice a high fat diet (HFD) increased hepatic glycogen, due to increased expression of the glycogenic scaffolding protein PTG/R5. PTG promoter activity was increased and glycogen levels were augmented in mice and cells after activation of mechanistic target of rapamycin complex 1 (mTORC1) and its downstream target sterol regulatory element binding protein 1 (SREBP1). Deletion of the PTG gene in mice prevented HFD-induced hepatic glycogen accumulation. Surprisingly, PTG deletion also blocked hepatic steatosis in HFD-fed mice, and reduced the expression of numerous lipogenic genes. Additionally, PTG deletion reduced fasting glucose and insulin levels in obese mice, while improving insulin sensitivity, a result of reduced hepatic glucose output. This metabolic crosstalk was due to decreased mTORC1 and SREBP activity in PTG knockout mice or knockdown cells, suggesting a positive feedback loop in which once accumulated, glycogen stimulates the mTORC1/SREBP1 pathway to shift energy storage to lipogenesis. Together, these data reveal a previously unappreciated broad role for glycogen in the control of energy homeostasis.