Aim: Long-distance dispersal (LDD) plays an important role in shaping the distribution of global biodiversity. Polyploidy could favour invasion and thereby facilitate LDD. However, how and to what extent polyploidy interacts with LDD remain unclear. Here, we test the putative role of polyploidy in the global dispersal of a cosmopolitan genus Rorippa. Location: Global. Time Period: Late Miocene to present. Major Taxa Studied: Rorippa Scop., Brassicaceae. Methods: We traced the biogeographical and speciation history for 17 diploids and 41 polyploids of Rorippa using variation from plastid genomes and multiple nuclear loci. The ploidy role in dispersal rate difference was demonstrated using trait-dependent biogeographical modelling. Results: LDD shaped the amphitropical disjunction of Rorippa, during which polyploids showed higher dispersal rates than those of diploids, with 5.6× increase under the best-fitted model. Five diploids and 21 polyploids were identified as products of transoceanic speciation events. Polyploidy-involved LDD was more common in terms of polyploidization following LDD than those preceding LDD. Main Conclusions: We demonstrate that polyploidy would be not only a driver but also a responder of LDD in Rorippa, highlighting a synergistic relationship between them. Our results provide a framework to uncover the biogeographical consequences of polyploidization and the joint roles of polyploidy and LDD in shaping the distribution of biodiversity.