<p>How nutrients impact plant allelopathy is elusive although allelopathy of invasive plants is greatly influenced by nutrient enrichment. However, the effect of potassium (K) on invasive plant allelopathy attracts less attention than those of other nutrients. We determined the allelopathy intensity of an invasive K-rich plant <i>Alternanthera Philoxeroides</i> (Mart.) Griseb. along a K fertilization gradient on seed germination and seedling growth of <i>Lac</i>tuca sativa var. <i>ramosa</i> Hort. and <i>Medicago sativa</i> L. and tried to discover the underlying mechanism. The results indicated that leaf extract showed inhibitory while root extract demonstrated stimulative allelopathic effects on the two studied species, which was attributed to the greater allelochemicals and K concentrations in leaves than in roots. Furthermore, the allelopathic effects increased along the K fertilization gradient. This might be associated with the observed increases in both the relative abundance of its allelochemicals and the tissue K concentration in response to K fertilization. These results suggested that the allelopathy intensity of <i>A. philoxeroides</i> could be promoted by K addition. This may be attributed to an enhanced production or exudation of allelochemicals, facilitated by the increased potassium availability for plant metabolism. Thus, regulating environmental K availability may offer a potential approach to weaken the allelopathic component of <i>A. philoxeroides</i> competitive ability. However, whether such a K-based management strategy can effectively contribute to field-scale ecosystem restoration needs to be tested in future studies.</p>