Rust is reported for the first time in North America on tansy (Tanacetum vulgare), a widespread and invasive Eurasian plant that was introduced into North America in the 17th century. Morphologically, the rust fungus corresponds exactly to Gäumann's description of Puccinia tanaceti, and to European specimens of P. tanaceti on T. vulgare. An inoculation study confirmed the narrow host specialization of P. tanaceti that Gäumann described, in that T. vulgare was successfully inoculated whereas four other species of the tribe Anthemidae of the Asteraceae were entirely resistant, i.e. Leucanthemum maximum (Shasta daisy), L. vulgare (ox-eye daisy), Artemisia absinthium (wormwood), and A. tridentata (sagebrush). This confirmation of a specialized P. tanaceti invalidates P. tanaceti s. lat. The latter complex has resulted from synonymies, and compilation of host records of rust fungi resembling P. tanaceti on at least 105 species representing 19 genera from six tribes of the Asteraceae from both New and Old Worlds.