Hepatic Notch signaling is inappropriately activated in obese/insulin-resistant mouse models. Genetic or pharmacologic inhibition of hepatic Notch signaling in obese mice simultaneously improves glucose tolerance and reduces hepatic triglyceride content. As such, we predicted that Notch signaling in human liver would be positively associated with insulin resistance and hepatic steatosis. Here, we systematically survey Notch signaling in liver biopsy specimens, and show active Notch signaling in lean and obese adults, with expression of multiple Notch receptors and ligands. In morbidly obese patients undergoing bariatric surgery, we show that Notch activation positively correlates with glucose-6-phosphatase (G6PC) and phosphoenolpyruvate carboxykinase (PCK1) expression, key regulators of hepatic glucose output. We used immunofluorescence to identify active Notch signaling in hepatocytes and show highest activity in hyperglycemia, which we confirmed is a direct effect of hyperglycemia and insulin resistance. In a validation cohort of leaner individuals undergoing percutaneous liver biopsy for suspected nonalcoholic fatty liver disease (NAFLD), Notch activity showed independent positive association with insulin resistance and hepatic steatosis. Notably, Notch activity showed stronger correlation with the NAFLD activity score and alanine aminotransferase levels than with steatosis alone, suggesting that Notch activity is associated with nonalcoholic steatohepatitis. In summary, this study establishes that Notch signaling is activated in and may represent a therapeutic target for patients with obesity-related liver disease.