A new species of Carex (Cyperaceae), Carex modesti, is described from southern Tanzania. It grows on stream sides and peat bogs at about 2750 m in the Kitulo Plateau. It is morphologically distinct from the similar species C. vallis-rosetto by its creeping rhizomes, coriaceous leaves and solitary spikes arising in each node. Carex modesti is included in Carex sect. Spirostachyae subsect. Elatae together with other Carex species from the tropical African mountains.