This study experimentally assesses the role of the endomicrobiome in the invasiveness of Taraxacum officinale (common dandelion), a widespread weed in Mediterranean ecosystems worldwide. The research compares the fitness of T. officinale from Mediterranean ecosystems on five continents grown with intact or reduced native seed-borne endomicrobiomes across five generations, and reports a competition experiment with F1 and F5 individuals assessing their impacts on native local Asteraceae species. T. officinale individuals harboring intact endomicrobiomes show faster and more favorable trait development compared with individuals with reduced endomicrobiomes. Enhanced competitiveness of endomicrobiome-colonised T. officinale plants with local Asteraceae species is apparently caused by increased synthesis of allelochemicals in shoots and rhizosphere soil, with gene expression analyses also showing the endomicrobiome to affect the expression by T. officinale of stress response and RNA-directed DNA methylation genes.