1. Invasive alien plants, together with organisms introduced for biological control, are ideal study systems with which to address questions of whether, and how fast, organisms adapt to changing environments. We compared populations of common ragweed, Ambrosia artemisiifolia, from native (USA) and introduced (China) ranges at similar latitudes, together with herbivores introduced for biological control, to understand the rate of evolutionary adaptive response of an invasive plant to novel environments. 2. Evolution of phenotypic traits associated with invasiveness was assessed by comparing differentiation in quantitative traits (QST) to that of neutral microsatellite genetic loci (FST) and through climate data. A common-garden experiment estimated quantitative genetic variation associated with competition with grasses and biological control history by beetles. 3. Three growth traits (height, total, and stem biomass) and plasticity associated with additional nutrients were significantly greater in invasive compared to native populations and differed from expectations from genetic drift alone. Native, but not invasive, populations exhibited traits showing evidence of past selection and correlations with climate, consistent with the recent timing of introductions. Competition experiments between invasive populations and a US bunch grass showed reduced competitive ability in populations with a history of biological control, that might indicate a trade-off between competitive ability and herbivore resistance in invasive populations. 4. Our results demonstrate the rapid rate at which traits favouring invasion can evolve in invasive weeds, such as A. artemisiifolia, but also that adaptation may reflect joint effects of release from specialist herbivores and novel climatic conditions.