Genetic diversity may promote a plant species’ invasiveness by facilitating the evolution of local adaptation, enhanced competitive abilities, and phenotypic plasticity. We examined the possible role of genetic diversity in the invasiveness of purple loosestrife, Lythrum salicaria L., a Eurasian native introduced to North America, through a broad geographic comparison of eight populations, four from Europe and four from North America. We further cultivated these populations in two common gardens varying by nearly 20 degrees of latitude to evaluate whether invasive populations exhibit heightened growth/reproductive capabilities and/or phenotypic plasticity when faced with widely varying environmental conditions. We found similar levels of genetic diversity regardless of continent of origin, indicating no loss of adaptive potential during the North American invasion, along with more rapid first-year growth of invasive populations regardless of garden. Further, we found evidence that our source populations had adapted to their respective latitudes on both continents, as plant size decreased with increases in a population’s latitude of origin. We observed no difference between continents in phenotypic plasticity in response to the widely varying conditions affecting our two gardens, suggesting either that such plasticity has played little role in the latitudinal spread of L. salicaria in North America, or that local selection has erased it over time. Our results support earlier studies demonstrating the evolution of local adaptation and increased competitiveness in invasive L. salicaria, bolstered by high levels of genetic diversity.