In this Conservation Action Plan, I evaluate the status of and make conservation recommendations for the four endemic primates of the Mentawai Islands: Kloss's gibbon (Hylobates klossii), the simakobu monkey (Simias concolor), the Mentawai langur (Presbytis potenziani), and the Mentawai macaque (Macaca pagensis). There are two subspecies of each of the cercopithecines. They are threatened mainly by hunting and commercial logging. This action plan follows up on the recommendations made for the Mentawai primates in the IUCN/SSC Primate Specialist Group's Action Plan for Asian Primate Conservation: 1987–91 by Ardith Eudey, and includes data and recommendations from researchers who have studied them since 1980. I also include a brief history of conservation action in the Mentawai Islands to put these recommendations in a historical context. I recommend the following conservation status changes: Kloss's gibbon, Endangered; simakobu monkey, Critically Endangered; Mentawai langur, Endangered; and Mentawai macaque, Vulnerable. The largest populations of the four species can be found in Siberut National Park on the largest and northernmost island, but all but Kloss's gibbon have distinct subspecies on the southern islands of Sipora, North Pagai, and South Pagai, and these are urgently in need of protective measures. The national park is remote and difficult to access by researchers and tourists. I suggest the following conservation actions: 1) increased protection for the Siberut National Park, which currently lacks enforcement; 2) formal protection of the Peleonan forest in North Siberut, which is home to unusually high primate populations and is easily accessible; 3) protect areas in the Pagai Islands by cooperating with a logging corporation that has practiced sustainable logging techniques there since 1971; 4) conservation education, especially regarding hunting; and 5) the development of alternative economic models for the local people to reduce the likelihood of selling off their lands to logging companies.