Aim Global climate change and increasing human disturbance are reshaping plant invasion patterns, yet it remains unclear how climate and disturbance interact to influence non-native plant diversity and how these effects are mediated by functional trait differences between native and non-native species. We aimed to quantify the joint and interactive effects of climate, human disturbance and trait differences on non-native plant diversity.Location Continental United States.Methods Using vegetation data from 47 terrestrial sites of the U.S. National Ecological Observatory Network (NEON), we analysed plot-level non-native plant diversity in relation to climatic gradients, human footprint index (HFP) and functional trait differences between co-occurring native and non-native species. Trait differences were quantified as plot-level contrasts in mean values of specific leaf area, plant height, leaf nitrogen content and seed mass, and were treated as explanatory variables rather than measures of diversity. Linear mixed-effects models were used to account for the hierarchical sampling design.Results Non-native plant diversity peaked at intermediate temperatures. Human disturbance altered moisture constraints on non-native plant diversity, reducing sensitivity to aridity gradients under high disturbance. Functional trait differences significantly mediated these patterns. In humid environments, disturbance favoured non-native species with relatively conservative traits (e.g., shorter height and lower specific leaf area), whereas in arid environments, non-native species with more acquisitive traits were more likely to establish.Main Conclusions Our results demonstrate that non-native plant diversity is shaped by context-dependent interactions among climate, human disturbance and functional trait differences between native and non-native species. By linking environmental gradients to trait-mediated invasion outcomes, this study provides trait-based insights into Darwin's naturalisation conundrum and improves predictions of plant invasions under ongoing global change.