We investigated the effect of a water-soluble extract from Grifola frondosa, the maitake mushroom, on lipid droplets in brown adipocyte tissue (BAT) cells. This water-soluble extract inhibits the conversion of pre white adipocyte tissue (WAT) cells but does not inhibit that of pre BAT cells. It reduces the amount of accumulated triglycerides (TG) in BAT cells. The glycerol-3-phosphate dehydrogenase (GPDH) activities of BAT cells decreased, but the expression of uncoupling protein 1 (UCP1) levels increased. These results suggest that maitake extract inhibits TG accumulation-related energy metabolism.