Fibroblast growth factor 21 (FGF21) is a metabolic regulator that provides efficient and durable glycemic and lipid control in various animal models. However, its potential to treat obesity, a major health concern affecting over 30% of the population, has not been fully explored. Here we report that systemic administration of FGF21 for 2 wk in diet-induced obese and ob/ob mice lowered their mean body weight by 20% predominantly via a reduction in adiposity. Although no decrease in total caloric intake or effect on physical activity was observed, FGF21-treated animals exhibited increased energy expenditure, fat utilization, and lipid excretion, reduced hepatosteatosis, and ameliorated glycemia. Transcriptional and blood cytokine profiling studies revealed effects consistent with the ability of FGF21 to ameliorate insulin and leptin resistance, enhance fat oxidation and suppress de novo lipogenesis in liver as well as to activate futile cycling in adipose. Overall, these data suggest that FGF21 exhibits the therapeutic characteristics necessary for an effective treatment of obesity and fatty liver disease and provides novel insights into the metabolic determinants of these activities.