Molecular mechanisms underpinning nonalcoholic fatty liver disease (NAFLD) are not well understood. The earliest step of NAFLD is hepatic steatosis, which is one of the main characteristics of aging liver. Here, we present a molecular scenario of age-related liver steatosis. We show that C/EBPα-S193D knockin mice have age-associated epigenetic changes and develop hepatic steatosis at 2 months of age. The underlying mechanism of the hepatic steatosis in old wild-type (WT) mice and in young S193D mice includes increased amounts of tripartite p300-C/EBPα/β complexes that activate promoters of five genes that drive triglyceride synthesis. Knockdown of p300 in old WT mice inhibits hepatic steatosis. Indeed, transgenic mice expressing dominant-negative p300 have fewer C/EBPα/β-p300 complexes and do not develop age-dependent hepatic steatosis. Notably, the p300-C/EBPα/β pathway is activated in the livers of patients with NAFLD. Thus, our results show that p300 and C/EBP proteins are essential participants in hepatic steatosis.