The Anopheles gambiae complex of mosquitoes includes malaria vectors at different stages of speciation, whose study enables a better understanding of how adaptation to divergent environmental conditions leads to evolution of reproductive isolation. We investigated the population genetic structure of closely-related sympatric taxa that have recently been proposed as separate species (An. coluzzii and An. gambiae), sampled from diverse habitats along the Gambia River in West Africa. We characterised putatively neutral microsatellite loci as well as chromosomal inversion polymorphisms known to be associated with ecological adaptation. The results revealed strong ecologically-associated population subdivisions within both species. Microsatellite loci at chromosome-3L revealed a clear differentiation between coastal and inland populations, which in An. coluzzii is reinforced by a peculiar inversion polymorphism pattern, supporting the hypothesis of genetic divergence driven by adaptation to the coastal habitat. Striking genetic differences, compatible with a strong reduction of gene-flow, were observed between An. gambiae populations west and east of an extensively rice-cultivated region exclusively occupied by An. coluzzii. Notably, this ‘intra-specific’ differentiation was higher than that observed between the two species and involved also the centromeric region of chromosome-X which has previously been considered a marker of speciation within this complex, suggesting that the two populations may be at an advanced stage of reproductive isolation triggered by human-made habitat fragmentation. These results confirm ongoing ecological speciation within these most important Afro-tropical malaria vectors and raise new questions on the possible effect of this process in malaria transmission.