High native species diversity generally suppresses non-native invasions, but many ecosystems are now characterized by non-native assemblages that vary in species diversity. How this non-native species diversity affects subsequent invaders and its environmental dependence remains unclear. Here, we conducted a plant-soil feedback experiment to investigate how non-native plant species diversity affects the growth of subsequent non-native plants, the role of soil microbes in this process, and the dependence of these patterns on drought. We found that under well-watered conditions, the biomass of subsequent invaders was higher with soil inocula generated by high non-native diversity, which was associated with higher arbuscular mycorrhizal fungal richness. However, under drought conditions, the biomass of subsequent invaders did not depend on soil inocula generated by non-native diversity. Our study reveals soil microbial legacies likely contribute to the commonly observed co-occurrence of multiple non-native species in nature and the importance of environmental conditions for these effects.