During November 2013, a few rust pustules (uredinia) were observed on the lower and upper sides of Euphorbia hirta leaves in Morelia (Michoacan state, Mexico). Uredinia were hypophyllous, golden-yellow, sub-epidermal erumpent and powdery, had a diameter of 0.3 to 0.5 mm and a height of 0.2 to 0.3 mm, and contained smooth- walled paraphyses intermixed with urediniospores. The heads of the paraphyses were clavate-capitate or capitate, 15.5 to 22.5 μm in diameter and had hyaline walls apically thickened up to 4 to 6 μm. Urediniospores were pale-yellow, pedicillate, variously shaped, as they included spherical, ellipsoidal and pyriform forms, measured 15-19x17- 23 μm, and showed hyaline echinulate walls 1.0 to 2.0 μm thick. A small number of subepidermal telial sori forming dark gray crusts were also found on the leaves close to the uredinia. Teliospores were one-celled, prismatic, 13-16×27-48 μm in size, had smooth, light coffee brown walls 1.0 to 1.5 μm thick, apically thickened up to 2 μm. Scanning electron microscopy observations revealed that urediniospores were globoid- or ellipsoid-shaped with fine echinulations. Based on the morphological characteristics and measurements, the rust was identified as Melampsora euphorbiae (Leon-Gallegos and Cummins, 1981). Samples were deposited in the Herbarium of the Faculty of Biology, Universidad Michoacana de San Nicolas de Hidalgo (UMSNH), Morelia, Michoacan, Mexico. To the best of our knowledge, this is the first record of M. euphorbiae on natural populations of E. hirta in Mexico.