Résumé: Delphinid populations show highly variable patterns of genetic diversity and population structure. Previous studies indicate that habitat discontinuities and geographic isolation are major drivers of population division in cetaceans. Spinner dolphins (Stenella longirostris) are distributed in all tropical oceans, but they are particularly common around islands and atolls. This species occurs in shallow waters at daytime to rest and socialise, and feeds on offshore mesopelagic prey overnight. Here, we investigated the genetic population structure of spinner dolphins in the Southwest Indian Ocean along a west-east geographic gradient, from eastern Africa to the Mascarene archipelago. We combined analyses of 12 microsatellite loci, mtDNA control region sequences, and sighting data to assess genetic differentiation and characterise habitat preferences of these populations. Significant genetic structure among the three sampled sites (Zanzibar, Mayotte and La Reunion) was observed using both types of molecular markers. Overall, our results indicate that geographic isolation and potentially other factors, such as shallow-water habitats to rest and socialise, may be important drivers of the genetic population structure of insular spinner dolphins in this region.