The polyphenolic compound theaflavin, the main red pigment in black tea, possesses many beneficial properties, such as fat-reducing and glucose-lowering capabilities. To produce theaflavin-containing fermentation water on a large scale, we have developed a simple, inexpensive, and selective enzymatic biotransformation method to obtain sufficient levels from fresh green tea leaves. Subsequent administration of theaflavin-containing fermentation water to obese mice on a high-fat diet inhibited body weight gain, decreased casual blood glucose and fasting blood glucose levels, and lowered mesenteric and total fat composition. To note, there were no significant differences observed in food consumption between the experimental and control (water without theaflavin) mice groups. Next, we investigated the effect of this water on blood glucose levels in healthy humans and found that it significantly inhibited blood glucose levels. Thus, we showed that theaflavin-containing fermentation water can be efficiently generated from fresh green tea leaves and demonstrated its significantly potent effects in vivo.