Description: Rapid adaptation to novel biotic interactions and abiotic factors in introduced ranges can be critical to invasion success of both exotic terrestrial and aquatic plants. Seaweeds are extremely successful biological invaders in marine environments. Along with herbivores, foulers − ubiquitous enemies in the marine environment − have the potential to determine the success or failure of invasive seaweeds. However, research on the topic of rapid adaptation of seaweeds to biotic challenges is still in its nascent stages and rapid adaptation of seaweeds to fouling is unexplored. We tested whether the impressive invasion success of the red macroalga Gracilaria vermiculophylla may be enhanced by the rapid adaptation of chemical control (defence) of new bacterial epibionts in the invaded range. The native and invasive G. vermiculophylla populations investigated were equally well defended against currently co-occurring bacterial epibionts isolated from their respective ranges. In contrast, the native populations were weakly defended against bacterial epibionts from the invaded range, whereas the invasive populations were weakly defended against bacterial epibionts from their native range. Apparently during the invasion process, invasive populations have adapted their control capacity to cope with the new epibionts but have lost the capacity to fend off old epibionts. Synthesis. These results provide the first evidence that a change in habitat and, thus, confrontation by new enemies, may trigger rapid defence adaptation of seaweeds, which could be necessary for invasiveness. Such adaptation dynamics as found in the current study could be also applicable to other types of host plant – enemy interaction e.g. plant root – microbe interactions, freshwater plant – fouler interactions in general and for cases of shifting plant – enemy interactions in course of climate change.