A key to the South American species of Winthemia Robineau-Desvoidy, 1830 is presented. 14 species, three of which described as new, are recognized: W. analis (Macquart, 1846); W. bicrucis (Townsend, 1932); W. brasiliensis (Townsend, 1927); W. ignobilis (Wulp, 1890); W. patagomca (Blanchard, 1963); W. picea (Walker, 1852); W. pinguioides (Townsend, 1934); W. singularis Reinhard, 1931; W. tricolor (Wulp, 1890); W. trinitatis Thompson, 1963; W. xanthocera (Wiedemann, 1830); W. augusta sp. n. (from Iguaçu, Paraná, Brazil); W. authentica sp. n. (from Cáceres, Mato Grosso, Brazil) and W. pandurata sp. n. (from Angra dos Reis, Rio de Janeiro, Brazil). The following new synonyms are established: Bicruciosturmia Townsend, 1932 = Winthemia R. - D 1830; Hemimasipoda alabamae Townsend, 1940 = W. aureonigra Thompson, 1963 = W. roblesi Valencia, 1972 = W. singularis Reinhard, 1931; Prowinthemia paraguayensis Townsend, 1928 = W. mina Reinhard, 1931 = W. tricolor (Wulp, 1890); W. reliqua Cortés & Campos, 1970 = W. trinitatis Thompson, 1963. The geographical distribution and illustration for all species are also provided.