The Bateke Plateau in the Republic of Congo is one of the last frontiers for ecology, with little known about its floristics and physiognomy. Despite occupying 89,800 km2 and its importance for local livelihoods, its ecology and ecosystem functions are poorly understood. Situated on Kalahari sands, the Bateke has a complex evolutionary history, mainly isolated from other savannas for much of its past, with currently unresolved ecological implications. Here we assess the biomass and floristic diversity of this savanna. We established four 25 ha permanent sample plots at two savanna sites, inventoried all trees, and assessed shrub, forb and grass species and biomass, and characterised the soils. Total plant carbon stocks (aboveground and belowground) were only 6.5 ± 0.3 MgC/ha, despite precipitation of 1600 mm/yr. Over half the biomass was grass, with the remainder divided between trees and shrubs. The carbon stock of the system is mostly contained in the top layer of the soil (16.7 ± 0.9 MgC/ha in 0-20 cm depth). We identified 49 plant species (4 trees, 13 shrubs, 4 sedges, 17 forbs and 11 grass species), with an average species richness of 23 per plot. There is tree hyperdominance of Hymenocardia acida (Phyllanthaceae), and a richer herbaceous species composition dominated by Loudetia simplex and Hyparrhenia diplandra. The low carbon stocks and tree biodiversity, compared to other African savannas, is surprising considering the high rainfall. We speculate it is due to low nutrient soils, high fire frequency and the effect of a temporally variable and restricted connection to the main southern African savanna complex.