Carpobrotus edulis (Aizoaceae) is a fleshy creeper, native to South Africa and commonly found growing on coastal seashores. Recently this plant has been observed dying in large patches in areas close to Cape Town. Symptoms include a wilting of the leaves associated with death of the woody stems. The aim of this study was to identify the probable cause of this disease. Dead and dying stem tissues were found to be colonised by a species of Cytospora. Isolates of this fungus were identified based on DNA sequence data from the rDNA-ITS, translation elongation factor 1-α, β-tubulin and large subunit rDNA loci. Analyses of the data showed that the fungus is a new species of Cytospora, described here as Cytospora carpobroti sp. nov. Pathogenicity tests showed that C. carpobroti resulted in distinct lesions on inoculated stems but not the fleshy leaves. The origin of C. carpobroti is unknown and there is concern that it could be an introduced pathogen threatening the health of this important native plant.