Catechins in green tea possess various health benefits. Enzymatic treatment improves physiological activities by inducing bioconversion of catechins. Here, we investigated the effect of green tea infusion (GT) after tannase treatment, which transforms (-)-epigallocatechin gallate (EGCG) to gallic acid (GA) and (-)-epigallocatechin (EGC), on adipocyte differentiation and mature adipocyte metabolism. The optimal conditions for tannase-mediated improvement in GA and EGC yields in GT were investigated using response surface methodology. Yields of GA and EGC were 43-fold (0.43 mg/mL) and 1.66-fold higher (1.11 mg/mL), respectively, compared to GT without tannase treatment. The optimal reaction conditions for tannase-mediated biotransformation were observed on 0.54 mg mL of tannase, reaction time (86.79 min), and reaction temperature at 22.59 °C. GT and tannase-treated GT (TANs) upregulated adiponectin, uncoupling protein 1, adipose triglyceride lipase, and hormone-sensitive lipase gene expression in differentiated 3T3-L1 adipocytes, with TAN inducing better effects than GT, which implies that tannase treatment improved the beneficial effect of GT on adipocyte metabolism. Thus, tannase-mediated bio-transformation is an attractive candidate for preparing GT with enhanced anti-obesity properties.