Six new species of the Spirobolida millipede genus Riotintobolus Wesener, 2009, are described from the spiny forest in southern Madagascar utilising genetic barcoding, drawings and scanning electron microscopy: Riotintobolus tsimelahy sp. nov., R. mangatsiaka sp. nov., R. lavanono sp. nov., R. bovinus sp. nov., R. antafoky sp. nov. and R. makayi sp. nov. One other Riotintobolus population from the spiny forest might represent an additional species based on genetic data, but it cannot be described as no male specimens were collected. At present, the genus Riotintobolus Wesener, 2009 has eight species from the spiny forest and two species from the littoral rainforest. A determination key to all ten species of the genus is provided. Molecular data reveal that the two critically endangered species from the humid littoral rainforest are not closely related to one another, but have their closest relative in the dry spiny forest ecosystem. Riotintobolus mandenensis Wesener, 2009, only known from the southern littoral rainforest of Mandena is related to R. tsimelahy sp. nov. from the nearby spiny forest at Tsimelahy with a p-distance of 11%, while R. minutus Wesener, 2009 from the littoral forest of Sainte Luce is more distant to all other Riotintobolus species, but more closely related to R. bovinus sp. nov. from the southwestern forest of the Makay.