Herein, we carried out the extraction of polyphenol resveratrol and its glycoside (piceid) from raw Japanese knotweed via a simple method, employing dimethyl ether (DME) as the solvent. With DME extraction, the conventional drying and cell wall disruption steps can be left out. As a result, more resveratrol and piceid were obtained in DME extraction than in conventional ethanol extraction. Because the Hansen solubility parameters of DME are closer to ethanol than to water, it allowed us to extract resveratrol and piceid from raw Japanese knotweed, unlike water. Simultaneously, 83.5% of the initial water and 213 mg/g dry of organic extracts were extracted from raw Japanese knotweed. As DME extraction does not involve drying and grinding of the decomposed antioxidants, it is highly suitable for extracting antioxidants.