The flora and biogeography of the Atlantic Sahara have been little studied, and most of the corresponding literature was published between 1935 and 1950. The paucity of data on the region reflects, in part, the extreme climate in much of the Atlantic Sahara, but also logistical and geopolitical obstacles. Many biogeographical questions remain unresolved. This paper presents an analysis of 22,000 vascular plant records for southern Morocco, northern Mauritania, and western Algeria. The floristic richness of the Atlantic Sahara (s.str.) is estimated at 578 taxa and subspecies, 78 being endemic (s.l.), yielding an endemism rate of 13 %. Biogeographic and bioclimatic analyses suggest that this area, positioned between the Mediterranean and Afro-tropical regions, forms an important gradient; the Atlantic Sahara may thus be best characterized as an ecotone or transition zone, as it was proposed by White in 1986 for the Sahara as a whole. Based on our geostatistical analyses of climate data and plant occurrences, and supported by recent collections and field observations, we propose a new phytogeographical bioregionalization for the Atlantic Sahara, including four new subregions, conforming to the framework of "Ecoregions of the World".