Temporal organization of tissue metabolism is important for maintaining nutrient and energy homeostasis in mammals. Autophagy is a conserved cellular pathway that is activated in response to nutrient limitation, resulting in the degradation of cytoplasmic components and the release of amino acids and other nutrients. Here, we show that autophagy exhibits robust circadian rhythm in mouse liver, which is accompanied by cyclic induction of genes involved in various steps of autophagy. Functional analyses of transcription factors and cofactors identified C/EBPβ as a potent activator of autophagy. C/EBPβ is rhythmically expressed in the liver and is regulated by both circadian and nutritional signals. In cultured primary hepatocytes, C/EBPβ stimulates the program of autophagy gene expression and is sufficient to activate autophagic protein degradation. Adenoviral-mediated RNAi knockdown of C/EBPβ in vivo abolishes diurnal autophagy rhythm in the liver. Further, circadian regulation of C/EBPβ and autophagy is disrupted in mice lacking a functional liver clock. We have thus identified C/EBPβ as a key factor that links autophagy to biological clock and maintains nutrient homeostasis throughout light/dark cycles.