In 2011, a distinct but unnamed form of titi, Plecturocebus, was rediscovered in the Chapada dos Parecis, a plateau in the southern extreme of the Brazilian state of Rondônia. Here we describe it as a new species based on an analysis of morphological and molecular traits. It can be distinguished from all other species of the genus Plecturocebus by its generally grayish agouti pelage. The limbs are grayish agouti on the outer surface, and grayish white on the inner surface; the breast, throat, and sideburns are also grayish white; the dorsum is reddish chestnut, and the tail is gray, paler to white at the tip. A phylogenetic analysis of the new species revealed a monophyletic clade with the four geographically closest species, and four scenarios of species delimitation indicated that speciation was recent. The geographic distribution of the new species is still poorly defined. Most of the recorded localities coincide with areas of higher elevation in the transition zone between the Amazon forest and the Cerrado savanna, in southern Rondônia and western Mato Grosso, including part of the Chapada dos Parecis (Parecis Plateau), and extending into the Aripuanã/Juruena and Aripuanã/Roosevelt interfluves. The range of the new species is in the area of the Amazon’s “Arc of Deforestation”, where enormous tracts of forest are being destroyed for timber, colonization and industrial agriculture. It occurs in the Juruena National Park and a few Indigenous protected areas. Based on the categories and criteria of the IUCN Red List, this new titi should be considered Near Threatened.