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<i>Acmella radicans</i> (Jacquin) R.K. Jansen is a new invasive species recorded in Yunnan Province, China, and little is known about its allelopathic potential and allelochemicals. In this study, the allelopathic effects of the essential oil (EO) of <i>A. radicans</i> on seed germination and seedling growth of four common plants, <i>Brassica napus</i>, <i>Brassica rapa</i> ssp. <i>chinensis</i>, <i>Digitaria sanguinalis</i>, and <i>Lolium multiflorum</i>, were explored. The results showed that the seed germination index, germination rate, root length, stem length, and biomass of <i>B. napus</i> and <i>B. rapa</i> ssp. <i>chinensis</i> were significantly inhibited at all EO concentrations of <i>A. radicans</i>, but there was a ‘low-promotion and high-inhibition effect’ on the root length of <i>D. sanguinalis</i> and <i>L. multiflorum</i> at low concentrations of 0.5 μL·mL⁻¹ and 0.5–1.0 μL·mL⁻¹, respectively. With increasing concentrations of EO, the inhibition rates of seed germination and seedling growth of four common plants gradually increased, and <i>D. sanguinalis</i> and <i>L. multiflorum</i> were the most inhibited, followed by <i>B. rapa</i> ssp. <i>chinensis</i>, and the least inhibited was <i>B. napus</i>. Thirty-two components were identified using GC–MS, representing 99.07% of the EO in <i>A. radicans</i>. The major components were 2-tridecanone (30.46%), caryophyllene oxide (19.18%), 4,8,11,11-tetramethylbicyclo[7.2.0]undec-3-en-5-ol (7.84%), β-caryophyllene (7.67%), and widdrol (4.7%). Among the compounds we identified, (E,E)-2,4-decadienal, 2-tridecanone, γ-cadinene, δ-cadinene, (E)-α-cadinol, spathulenol, caryophyllene oxide, and widdrol have been previously reported as having possible allelopathic effects. Our study was the first to show that <i>A. radicans</i> could potentially release allelochemicals to influence neighboring plants during its invasion and expansion.