The world is experiencing increasing climatic variability, an ongoing loss of biodiversity and a growing spread of invasive species. Previous experimental studies demonstrated that the invasibility of plant populations is reduced with increasing resident genetic diversity and is promoted by environmental fluctuations, but their combined effect has so far not been considered. In a growth chamber experiment, we tested whether the genotypic diversity of experimental populations of Arabidopsis thaliana (1, 3 or 6 genotypes) and temperature fluctuations affect population invasion by Senecio vulgaris, and how these factors interact. We found that genotypic diversity tended to increase the invasion resistance of experimental plant populations in terms of the biomass ratio between the species, and that temperature fluctuations strongly favoured Arabidopsis (biomass: +49%) over Senecio (–28%). However, there were no interactions between environmental fluctuations and genotypic diversity. Nevertheless, the magnitude of net diversity effects and transgressive overyielding depended on temperature conditions, indicating that increased environmental variability can influence diversity mechanisms. Our study shows that, although genotypic diversity and environmental variability did not interact, these two factors independently affected the invasibility of plant populations.