| Summary: | <i>Reynoutria japonica</i> Houtt. is a critical invasive alien plant in Europe and North America with a drastic impact on native flora. However, <i>R. japonica</i> has medicinal potential, especially as a source of stilbenes. In order to explore the potential of simple extractions of <i>R. japonica</i>, we conducted qualitative and quantitative analyses of fresh <i>R. japonica</i> rhizome infusion, decoction, and macerates with ethanol by UHPLC-DAD-ESI-MS<sup>n</sup> and UHPLC-DAD, with a focus on major constituent groups of stilbenes and anthranoids. Since <i>R. japonica</i> rhizome extracts showed antimicrobial potential in the past, we also evaluated the antimycobacterial effect of raw <i>R. japonica</i> extracts for the first time against <i>Mycobacterium smegmatis</i>. Of thirty-four characterized substances, six were stilbenes and twelve anthranoids. The main constituents, four trans-stilbenes and eight anthranoids, were quantified in a validated UHPLC-DAD method. The 38% ethanol macerate showed high stilbene (155.078 mg/100 g fluid extract) and low anthranoid content (5.420 mg/100 g fluid extract), while decoction showed the highest anthranoids. Antimycobacterial testing gave good results for all macerates (MIC 256 µg/mL) and <i>trans</i>-resveratrol (64 µg/mL). Extraction and enrichment of stilbenes from fresh plant material by simple extraction methods with food-grade solvents might encourage consideration of wild harvest of rhizomes over classic means of eradication of <i>R. japonica</i>.
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