Increased de novo lipogenesis is a hallmark of metabolic dysfunction-associated steatotic liver disease (MASLD) in obesity, but the macronutrient carbon source for over half of hepatic fatty acid synthesis remains undetermined. Here, we discover that dietary protein, rather than carbohydrates or fat, is the primary nutritional risk factor for MASLD in humans. Consistently, ex vivo tracing studies identify amino acids as a major carbon supplier for the tricarboxylic acid (TCA) cycle and lipogenesis in isolated mouse hepatocytes. In vivo, dietary amino acids are twice as efficient as glucose in fueling hepatic fatty acid synthesis. The onset of obesity further drives amino acids into fatty acid synthesis through reductive carboxylation, while genetic and chemical interventions that divert amino acid carbon away from lipogenesis alleviate hepatic steatosis. Finally, low-protein diets (LPDs) not only prevent body weight gain in obese mice but also reduce hepatic lipid accumulation and liver damage. Together, this study uncovers the significant role of amino acids in hepatic lipogenesis and suggests a previously unappreciated nutritional intervention target for MASLD.