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Previous genetic studies of frogs from Mayotte Island (a French Overseas Department in the Comoros Archipelago) in the Western Indian Ocean have provided evidence for oceanic dispersal in amphibians, which is a rare phenomenon due to the osmotic intolerance of amphibians to saline water. Using an integrative approach including morphological, bioacoustic, and genetic evidence, we here confirm that these frogs correspond to two new species and are the only representatives of the family Mantellidae not endemic to Madagascar. Blommersia transmarina sp. nov. differs from its sister taxon, B. wittei, by several morphological differences including larger body size (snout-vent length up to 34.5 mm) and by slight differences in advertisement calls. Boophis nauticus sp. nov. differs from its closest relatives, B. tephraeomystax and B. doulioti, by slight morphological differences (including larger body size), a reddish (vs. silvery or golden) iris coloration in life, and slightly different advertisement calls. The two new species differ from their closest relatives by a substantial genetic differentiation, with pairwise genetic distances > 5% in the mitochondrial 16S rRNA gene, and based on the limited available data, also by distinct differences in nuclear DNA. They also are both larger than their closest relatives from Madagascar and B. transmarina sp. nov. is the largest Blommersia species, suggesting a moderate form of island gigantism. The Madagascan sister species B. wittei and B. doulioti are among the relatively few amphibian species occurring in the arid western biomes of the island, are adapted to open landscape, and reproduce in stagnant water bodies, which we hypothesizemay represent important preadaptations for successful overseas colonization.