Tilletia walkeri (Ustilaginales: Tilletiaceae) is described as a new species of partial bunt infecting Lolium multiflorum, annual ryegrass, and L. perenne, perennial ryegrass in the United States and Australia, respectively. The new species is characterized by large, tuberculate teliospores with the exospore ornamentation comprised of incompletely cerebriform ridges in surface view. Teliospores of T. walkeri are compared with those of T. indica and other similar species of Tilletia, and the issue of Neovossia versus Tilletia is discussed. A key is provided to smuts known to occur on species of Lolium.