Alluaudia is a genus of six species of flowering plants endemic to Madagascar, where they form an important component of the Madagascar spiny forests. They are spiny succulent shrubs and trees from 2–20 m tall, with leaves that are deciduous in the long dry season. The leaves are in pairs, 0.5-3.5 cm long, with one or two thorns 2-2.5 cm long in the axil of each pair of leaves. The flowers are small, grouped many together in large numbers.
Several of the species are grown as indoor ornamental plants in specialist succulent collections. Of the six species Alluaudia ascendens, A. procera, A. montagnacii, and A. dumosa form a tall central trunk that that sprouts strong vertical branches. Alluaudia comosa and A. humbertii are shorter, and form a thick tangle of horizontal branches. Alluaudia dumosa, unlike the other species, bears small leaves only on the new growth and are quickly shed. Photosynthesis is carried on by the stems.