We evaluated total phenolic content, antioxidant activity, reducing power and antibacterial activity of ethanol, hexane, chloroform, ethyl acetate and aqueous extracts of aerial parts of Rumex japonicus HOUTT. The ethyl acetate extract had the highest amount of phenolic compounds. It also exhibited the highest reducing power and antioxidant activity when assayed by the 1,1-diphenyl-2-picrylhydrazyl (DPPH), β-carotene bleaching and superoxide radical methods. The ethyl acetate extract possessed the strongest antibacterial activity against Bacillus subtilis, B. cereus and E. coli. GC-MS analysis indicated that ethyl acetate extract contained a variety of phenolic compounds. HPLC analysis showed that pyrogallol was the predominant phenolic compound in this extract. Thus, our study verified that the ethyl acetate extract has strong antioxidant and antibacterial activities which are correlated with its high level of phenolic compounds, particularly pyrogallol and pyrocatechin. This extract of R. japonicus aerial parts can be utilized as an effective and safe source of antioxidants.