Loss of skeletal muscle mass and function with age represents an important source of frailty and functional decline in the elderly. Antioxidants from botanical extracts have been shown to enhance the development, mass, and strength of skeletal muscle by influencing age-related cellular and molecular processes. Tannase-treated green tea extract contains high levels of the antioxidants (−)-epicatechin (EC) and gallic acid that may have therapeutic benefits for age-related muscle decline. The aim of this study was to investigate the effect of tannase-treated green tea extract on various muscle-related parameters, without concomitant exercise, in a single-center, randomized, double-blind, placebo-controlled study. Administration of tannase-treated green tea extract (600 mg/day) for 12 weeks significantly increased isokinetic flexor muscle and handgrip strength in the treatment group compared with those in the placebo (control) group. In addition, the control group showed a significant decrease in arm muscle mass after 12 weeks, whereas no significant change was observed in the treatment group. Blood serum levels of follistatin, myostatin, high-sensitivity C-reactive protein (hs-CRP), interleukin (IL)-6, IL-8, insulin-like growth factor-1 (IGF-1), and cortisol were analyzed, and the decrease in myostatin resulting from the administration of tannase-treated green tea extract was found to be related to the change in muscle mass and strength. In summary, oral administration of tannase-treated green tea extract containing antioxidants without concomitant exercise can improve muscle mass and strength and may have therapeutic benefits in age-related muscle function decline.