A cruciferous weed species, Rorippa sylvestris Besser, has been naturalized in Hokkaido since the 1950's and is one of the most notorious weeds in wettish fields and pastures, with strong propagative power and potential allelopathic effect. Roots of R. sylvestris were extracted with ether, and the neutral fraction of the ether extracts inhibited lettuce (Lactuca sativa) seed germination. This fraction was subjected to silica gel column chromatography (eluted with EtOAc-MeOH 1:1) and preparative TLC, yielding the major inhibitory principle as a yellowish oil. The high-resolution mass spectrum suggested its molecular formula C₁₀H₁₉ONS₂, and ¹H-NMR (100 MHz) and IR spectral data indicated it was 8-methylsulfinyloctyl isothiocyanate (1), whose structure was further confirmed by synthesis. Biological activity tests showed compound 1 completely inhibited lettuce seed germination at 200 ppm, reduced root and hypocotyl elongations of lettuce seedlings by 86% and 90% respectively at 4×10⁻⁴ M (93.2 ppm), and affected root growth even at 10 ppm. Previously isolated from Arabis hirsuta seeds, its plant growth inhibitory activity was unreported before. This study points out that 8-methylsulfinyloctyl isothiocyanate (1) is an allelochemical candidate from R. sylvestris with both antimicrobial and significant plant growth inhibitory activities.