The study on recently collected specimens of Populus x euramericana infected by a Melmpsora species revealed that the specimens are different from other Melampsora species recorded on Populus in Iran. Uredinia and telia were found on the infected plant. Uredinia hypophyllous, scattered, pulverulent, and round often covered the whole leaf surface, urediniospores were mostly obovoid, 31-43x15-20 mm, their wall echinulate, smooth at apex, 1.5-2 mm thick at apex, up to 6 mm thick at sides, and paraphyses were clavate, their wall up to 13 mm thick at the apex. Telia was black, amphigenous but mostly epiphyllous, sub epidermal, teliospores adhered laterally, 40-64x8-12 mm, wall 1 mm thick, apically thickened up to 2.5 mm. Fungal morphological features matched features characteristic of Melampsora larici-populina Kleb. This is the first report of a rust disease attacking leaves of Populus x euramericana in Iran. Melampsora laricipopulina is also newly reported for the Iranian Rust mycobiota.