A new smut fungus from Namibia, Tilletia triraphidis (Tilletiaceae) on Triraphis purpurea (Poaceae: Triraphideae), is described and illustrated. It differs from all other species of Tilletia by specialization on Triraphis. Tilletia triraphidis is distinguished from the known smut fungi on host plants in the tribe Triraphideae (Ustilago neyraudiae, U. latzii, and U. triraphidis) by having sterile cells and reticulate spores. A key to the smut fungi that infect Neyraudia and Triraphis is also provided. Keywords: Namibia, Poaceae, smut fungi, taxonomy, Tilletia, Tilletiaceae, Triraphis, Fungi.