<p>Competitive interactions during community assembly are critical for suppressing invasive species and restoring native biodiversity. However, the roles of plant community attributes in determining competitive suppression remain unclear. We examined the competitive suppression of the invasive perennial herb <i>Solidago altissima</i> L. through a two-year greenhouse experiment. To assess species identity effects, we established pairwise competitions between <i>S. altissima</i> and 19 native species. Diversity effects were evaluated by comparing invasion success in eight four-species native mixtures versus monocultures using relative competition indices (<i>RCI</i>) based on height, cover, density, and biomass. Species identity and functional groups were critical determinants of competitive outcomes. 14 native species significantly suppressed <i>S. altissima</i> growth, with annual herbs (<i>RCI</i> = 0.608–0.771) and perennial herbs (<i>RCI</i> = 0.471–0.666) providing the strongest suppression, while woody perennials were the least effective (<i>RCI</i> = 0.197–0.250). Species diversity showed marginally positive but non-significant effects. Competitive suppression of <i>S. altissima</i> was marginally stronger in mixtures than in monocultures by the second year (mixture <i>RCI</i> = 0.764, monoculture <i>RCI</i> = 0.531, <i>P</i> = 0.084). Functional traits showed inconsistent relationships with competitive outcomes, and trait similarity did not predict invasion suppression. Our results indicate that competitive suppression is primarily driven by species and functional group identities rather than by diversity or trait similarities. Restoration efforts should prioritize fast-growing, canopy-covering non-woody native annuals and perennials—particularly forbs with high specific leaf area, large seed mass, and early canopy dominance—to maximize competitive suppression during community assembly.</p>