Saharan dust that is transported over the Atlantic Ocean provides an important input of phosphorus (P) to the oligotrophic waters of ocean and the P-depleted rain forests of America. To establish more firmly the role of Saharan dust events as P suppliers, the dust P sources needs to be identified. From analysis of phosphate oxygen isotopes of all the major Saharan dust events of 2011 over the Cape Verde islands in the northeastern Atlantic, supported by remote-sensing imagery, we infer that the dust P originates from widespread sedimentary sources and magmatic P “hot spots”, in which the latter enrich the dust in bioavailable P. The fraction contributed from each source varied markedly between dust events. We also found that phosphate from the Bodélé depression is not evident in Cape Verde. Our results provide new information for global biogeochemical studies and suggest that identification of Saharan dust P sources is required to improve their accuracy.