Obesity is caused by an imbalance between caloric intake and energy expenditure and accumulation of excess lipids in adipose tissues. Recent studies have demonstrated that green tea and its processed products (e.g., oolong and black tea) are introduced to exert beneficial effects on lipid metabolism. Here, we propose that fermented green tea (FGT) extract, as a novel processed green tea, exhibits antiobesity effects. FGT reduced body weight gain and fat mass without modifying food intake. mRNA expression levels of lipogenic and inflammatory genes were downregulated in white adipose tissue of FGT-administered mice. FGT treatment alleviated glucose intolerance and fatty liver symptoms, common complications of obesity. Notably, FGT restored the changes in gut microbiota composition (e.g., the Firmicutes/Bacteroidetes and Bacteroides/Prevotella ratios), which is reported to be closely related with the development of obesity and insulin resistance, induced by high-fat diets. Collectively, FGT improves obesity and its associated symptoms and modulates composition of gut microbiota; thus, it could be used as a novel dietary component to control obesity and related symptoms.