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Seasonally Dry Tropical Plant Formations (SDTFs) are among the most threatened biomes worldwide. Nevertheless, they have received less scientific attention than rainforests or savannas. One such SDTF, the Caatinga Phytogeographic Domain (CPD), is the largest semiarid tropical ecoregion in South America. Earlier floristic studies of the Caatinga flora discerned two floristic groups in the CPD, in terrains of crystalline and sedimentary origin respectively. We compiled and analysed the most comprehensive dataset on CPD plant distributions to produce a general biogeographical synthesis for Caatinga, including inselbergs, riverine forests and ecotonal areas. Not only are crystalline and sedimentary caatingas clearly distinct, but inselbergs collectively form a third floristic group in the CPD. Non-woody plants, omitted from many studies, are a major component of Caatinga diversity, comprising more than 60 % of species in some local communities. Raunkiaerian life-form spectra of Caatinga sites show that Caatinga vegetation differs in life-form spectra from those of major world biomes, supporting recognition of SDTFs as a discrete world biome.