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Plant invasions and drought are global change stressors that alter plant availability and quality, habitat structure, and micro‐site conditions, with likely cascading effects on arthropod diversity, yet few studies have disentangled their relative impacts. We quantified the effects of experimental plant invasion (by cogongrass Imperata cylindrica ) and simulated drought, imposed singly and in combination, on abundance and morphospecies richness of arthropod communities, functional groups, and individual taxa in regenerating longleaf pine understory located in humid, subtropical central Florida, USA. Across all three scales of organization, invasion emerged as a negative stressor, both alone and in combination with drought, while drought alone had mostly positive effects on arthropods. At the community scale, arthropod abundance and richness were 74 and 55% lower, respectively, in invaded than uninvaded plots (averaging over drought) two years after initiation of invasion, while drought had no negative effect. Herbivores, pollinators, and predators had lower abundances (by 78, 48 and 73%, respectively) and richness (by 63, 29 and 67%) in invaded than uninvaded plots by the second year of invasion. Herbivorous hemipterans, caterpillars, and beetles were more negatively affected by invasion than grasshoppers, while for pollinators, flies (rather than moths or bees) drove the negative response, exhibiting lower abundance and richness when both stressors were present. There was also a cascading negative effect of invasion on the abundances of predatory wasps and spiders, and the richness of predatory wasps. Conversely, several disparate taxa responded positively to drought without invasion, despite significantly warmer and drier conditions under drought. The pronounced negative effect of invasion on arthropod communities was driven partly by its reduction of resident plant richness, which was in turn positively correlated with arthropod abundances and richness. These results suggest plant invasion as one important driver of arthropod decline in a rapidly changing world.