Dodder is a parasitic weed that is troublesome to the growth of many plants. Our study shows that this invasive species contains strong allelopathic potential, exerting strong inhibition against the growth of indicator plants and noxious paddy weeds in bioassay and pot trials. In a greenhouse, incorporation of 0.5 t ha−1 of dried dodder plants to paddy soil reduced spontaneous growth of paddy weeds by about 50%, whereas the 1.5 to 2 t ha−1 dose suppressed biomass of paddy weeds by more than 75% and completely controlled emergence of barnyardgrass and monochoria. By the use of a separation resin, 22 compounds were separated from dodder and identified by gas chromatography–mass spectrometry as belonging to terpenes, long-chain fatty acids, phenols, phenolic acids, and lactone. Among these compounds, 15 substances were quantified and tested for their herbicidal activity. Quantity of cinnamic acid was the highest (37.3 mg g−1), followed by dihydro-5,6-dehydrokavain (DDK; 6.0 mg g−1), myristic acid (3.2 mg g−1), and methyl cinnamate (2.1 mg g−1), whereas the amounts of other compounds were between 0.01 and 0.1 mg g−1. It is suggested that the content of the terpenes within dodder, which was rather high in amount (0.41–2.1 mg g−1), correlated to its strength of chemical cues to find host plants. Cinnamic acid, DDK, methyl cinnamate, and vanillin exerted the most potent herbicidal activities against radish growth. Findings of this study propose that cinnamic acid, DDK, and methyl cinnamate are responsible for its strong phytotoxic action of dodder plants. However, whether these plant growth inhibitors and other compounds detected from the dodder can suppress emergence of their hosts as well as contributing to its strong invasiveness needs further elucidation.