Abstract. Raft spiders, family Dolomedidae, represent a group of spiders with 7 genera distributed near globally well known for a semi-aquatic lifestyle. One of the iconic genera representing such a unique lifestyle is the largest and diverse dolomedid genus Dolomedes. Although Dolomedes is well surveyed in parts of its distribution, its diversity in the tropics remains largely unknown, such as the western Indian Ocean region with most of the islands unsurveyed. Recent expeditions to Madagascar and La Réunion Island discovered a series of specimens that appear to belong to an unknown morphospecies of Dolomedes, exhibiting varying degrees of differences in the diagnostic characteristics. To further test and define their species boundaries, we include molecular evidence followed by species delimitation analyses. Our findings indicate that specimens from both islands are conspecific. Consequently, we introduce a new species to science, Dolomedes reuniascar sp. nov., providing detailed illustrations, descriptions of its variations and ecological characteristics. Based on our observations and existing literature, we speculate that the presence of this species on the two isolated islands could be attributed to either highly efficient dispersal abilities, such as ballooning of spiderlings, or human-mediated introduction events.