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Virus infections have the potential to reduce biomass yields in energy crops, including Panicum virgatum (switchgrass). As a first step towards managing virus-induced biomass reduction, deep sequencing was used to identify viruses associated with mosaic symptoms in switchgrass. Two sequences with homology to mastreviruses were identified. Total DNA extracted from switchgrass varieties 'Dewey Blue' and 'Cloud Nine' was used as template to amplify mastrevirus DNA by the rolling-circle method. Complete mastrevirus genome sequences were obtained from cloned amplicons. The two nucleotide sequences were 88 % identical to each other but only 56-57 % identical to the closest relatives in the genus Mastrevirus. Predicted amino acid sequences of the coat protein, replication-associated protein A, replication-associated protein, and putative movement protein encoded by the two mastrevirus-like sequences were 95 %, 79 %, 79 %, and 87 % identical to each other, respectively, and 46-48 %, 31 %, 31 %, and 42-48 % identical to those of the closest mastrevirus relatives. Based on a genome-wide identity threshold of 75 % set by the International Committee on Taxonomy of Viruses and phylogenetic analyses, the two virus sequences appear to represent a new mastrevirus species. The mastrevirus is tentatively named switchgrass mosaic-associated virus 1 (SgMaV-1) and is the first mastrevirus reported from North America.