A virus was isolated from joyweed (Alternanthera tenella Colla-Amaranthaceae), a common weed in tropical and sub-tropical regions. Examination by electron microscopy showed long flexuous particles with an average length of 756 nm in crude sap. Serological results showed positive reaction with antisera to PVY-O. A fragment of 1772 nucleotides was sequenced. The CP sequence shares 76% of identity with the CP of Potato virus Y strain NTN. These results confirm that the virus is a new potyvirus infecting A. tenella, and the name Alternanthera mild mosaic virus (AltMMV) is proposed.