A novel dwarf and twisting syndrome first observed on rice in Nghe An Province, Vietnam, in 2009 has spread rapidly to the other 19 provinces of North and Central Vietnam. Infected rice plants showed stunting, darkening of leaves, twisting of leaf tips, and splitting of leaf margins. At a later stage, white waxy enations that eventually turned black were observed on the underside of leaf blades, leaf sheaths, and culms. The disease also infected maize after rice was harvested. Infected maize plants were stunted and dark green with small enations along the minor veins on the back of leaves. The disease agent has now been identified as Southern rice black-streaked dwarf virus (SRBSDV) recently reported from Southern China. Typical fijivirus viroplasms containing crystalline arrayed spherical virions approximately 70 to 75 nm in diameter were observed under the electron microscope in ultrathin sections of infected rice leaves. The virus was transmitted to rice and maize seedlings by the white-backed planthopper (Sogatella furcimera). A one-step reverse transcription-polymerase chain reaction (RT-PCR) protocol was used to confirm the presence of SRBSDV in 477 samples of rice or maize from 29 provinces among 5 agroecological regions in North and Central Vietnam. Rice black-streaked dwarf virus was not detected in these samples. Partial sequences of RNA segments 4 and 10 from several isolates showed very low genetic divergences between isolates from Vietnam and China, suggesting a common origin, and phylogenetic analysis confirmed the placement of SRBSDV as a distinct virus within subgroup 2 of the genus Fijivirus.