The Arctic is warming four times faster than the global average, and plant communities are responding through shifts in species abundance, composition and distribution. However, the direction and magnitude of local plant diversity changes have not been quantified thus far at sites across the Arctic. Using a compilation of 42,234 records of 490 vascular plant species from 2,174 plots at 45 study areas across the Arctic, we quantified temporal changes in species richness and composition from repeat surveys conducted over different intervals between 1981 and 2022, and identified the geographic, climatic and biotic drivers behind these changes. We found greater species richness at lower latitudes and warmer sites, but no indication that local species richness was changing directionally over time, on average. However, species turnover was widespread, with 59% of plots gaining and/or losing species. Proportions of species gains and losses were greater where temperatures had warmed most. Shrub expansion, particularly of erect shrubs, was associated with greater species losses and decreasing richness. Despite changes in plant composition, Arctic plant communities did not become more similar to each other over time, suggesting that no biotic homogenisation has occurred thus far. Overall, we found that Arctic plant communities changed in richness and composition in different directions, with temperature and plant-plant interactions emerging as the main drivers of directional change. Our findings demonstrate how climate and biotic drivers can act in concert to alter plant composition, which could be the precursor of future biodiversity change with impacts on ecosystem function, wildlife habitats and livelihoods for Arctic people.