The "Critically Endangered" southern patas monkey Erythrocebus baumstarki, thought to be endemic to Tanzania, has been resurrected to species level based on its geographic isolation, and on the coloration and pattern of its pelage. This study presents the first evidence for E. baumstarki in Kenya and reviews its historic and current geographic distributions based on the literature, museum specimens, online platforms, responses to requests for site records, and our own fieldwork. The distribution of E. baumstarki in the early 20th century was roughly 66,000 km2 . This has declined about 85% to around 9700 km2 at present (post-2009). The current "Extent of Occurrence" is only about 2150 km2 . This species was extirpated from Kenya in about 2015 and from the Kilimanjaro Region in Tanzania in about 2011. At present, E. baumstarki appears to be restricted to the protected areas of the western Serengeti, with the western Serengeti National Park being the stronghold. The number of individuals remaining is probably between 100 and 200, including between 50 and 100 mature individuals. The ultimate threat to E. baumstarki is the very rapidly increasing human population, while the main proximate threats are the degradation, loss, and fragmentation of natural habitats, and the related competition with people and livestock for habitat and water, particularly during droughts. Other problems are hunting by poachers and domestic dogs, and probably loss of genetic variation and climate change. This article provides recommendations for reducing the threats and promoting the recovery of E. baumstarki. We hope this article heightens awareness of the dire conservation status of E. baumstarki and encourages an increase in research and conservation action for this monkey.