The Afrotropical freshwater crab genus Seychellum is endemic to the granitic Seychelles in the Indian Ocean (Mahé, Silhouette, Praslin, La Digue and Frégate). Here we describe two new cryptic species of Seychellum that represent two evolutionarily separate lineages of a previously monotypic genus. This raises to three the number of species of freshwater crabs known from Seychelles. Each species is endemic to either one island (Silhouette) or to a pair of islands (Mahé and Frégate, or Praslin and La Digue). The three species can be clearly distinguished as separate lineages by DNA analysis, haplotyping and examination of gonopod characters. The recognition of S. silhouette, sp. nov. (endemic to Silhouette) and S. mahefregate, sp. nov. (endemic to Mahé and Frégate) reduces the range of the type species, S. alluaudi (A. Milne-Edwards & Bouvier, 1893) to La Digue and Praslin. Both dispersal and vicariance may have played a role in shaping the present distribution patterns of the Seychellois freshwater crabs.