Senecio cadiscus is a species of aquatic flowering plant in the aster family, Asteraceae. It is endemic to the Western Cape of South Africa, where it grows in vernal pools. It was at one time classified as Cadiscus aquaticus, the only species in the monotypic genus Cadiscus, but molecular phylogenetic analysis showed that it is nested within Senecio. It is a floating or emergent plant with small narrow leaves and white-rayed flowers. Its populations are small and fragmented, and are declining. The plant is threatened by grazing and trampling by livestock, invasive plants, reclamation of wetlands and eutrophication from fertilizer runoff. The International Union for Conservation of Nature has consequently rated it as "critically endangered".