The prevalence of metabolic diseases has risen globally in parallel with the obesity epidemic over the past few decades. Green tea has been reported to have metabolically beneficial effects on obesity; however, the mechanism by which green tea regulates lipid metabolism is not clearly understood. Male c57BL/6 mice were fed a normal chow diet, a high-fat diet (HFD), or an HFD supplemented with various doses of epigallocatechin gallate-rich green tea extract (GTE) for 12 weeks. GTE supplementation reduced body weight gain, prevented hepatic fat accumulation, decreased hypertriglyceridemia, and improved hyperglycemia and insulin resistance in HFD-fed mice. The underlying mechanisms of these beneficial effects of GTE might involve the upregulation of sirtuin 1 and AMP activated protein kinase (AMPK) and the downregulation of enzymes related to de novo lipogenesis. Consistent with the in vivo findings, GTE increased the expression and activity of sirtuin 1, enhanced the binding of sirtuin 1 to liver kinase B1 (LKB1) and subsequent deacetylation of LKB1, and reduced triglyceride accumulation in HepG2 cells. These results suggest the possible therapeutic potential of dietary epigallocatechin gallate-rich GTE supplementation for preventing the development and progression of hepatic steatosis and obesity.