Euphorbia otavimontana Swanepoel, here described as a new species, is a robust, spiny, succulent shrub endemic to the Otavi Mountains and surrounding areas in northern Namibia. It is probably most closely related to E. ingenticapsa from western Angola. Diagnostic characters for the new species include the variably shaped stem segments with greyish white or grey margins, the obconic involucre with the glands spreading in the central initial cyathium and erect or suberect in lateral cyathia, the papillate bifid styles and the obtusely trigonous or 3-lobed capsule which lacks turgid sinuses when developing. A comparative table with diagnostic morphological features to distinguish between E. otavimontana and E. ingenticapsa is provided. Based on IUCN Red List categories and criteria, a conservation assessment of "Endangered" is recommended for the new species. Keywords: Euphorbieae, Euphorbia otavimontana, Karstveld, limestone, new species, Namibia, Otavi Group, succulents, taxonomy.