Green tea is reported to exert beneficial effects on metabolic disorders through the regulation of lipid metabolism. On the contrary, fermented food products have been introduced to improve human health by modulating immune response and energy metabolism. To maximize health benefit, we applied fermentation processing to green tea. Fermented green tea extract (FGT) inhibited adipogenesis and lipogenesis in cultured adipocytes, whereas it augmented mRNA expression of fatty acid oxidation-related genes in differentiated myocytes. In diet-induced obese mice, FGT blunted body weight and fat mass gain by 69.7% and 56.7%, respectively. FGT also improved circulating triglyceride concentrations by 32.6%. Similar to in vitro results, FGT suppressed lipogenesis and promoted lipid catabolism in peripheral tissues. In addition, FGT administration modulated the composition of certain gut microbiota which are associated with obesity and related metabolic disorders. Among the various components of FGT, gallocatechin gallate is suggested to mediate the effect of FGT on lipid metabolism. Taken together, we propose FGT as a novel functional food to benefit human health by controlling adiposity and lipid metabolism.