Phytophthora infestans (Mont.) de Bary is the oomycete that caused the historic Irish potato famine of 1845-1850 and beyond, and it continues to cause worldwide devastation of the modern potato and tomato industries. Double-stranded RNAs (dsRNAs) have been discovered in P. infestans but have never been further investigated. Our lab has discovered several dsRNAs that are effectively viral genomes. One of these viruses has been characterized and is tentatively named Phytophthora infestans RNA virus 4 (PiRV-4). PiRV-4 has a 3.00 kb genome with one open reading frame (ORF) coding for a supergroup 2 RNA-dependent RNA polymerase (RdRp). Based on sequence and phylogenetic comparisons, PiRV-4 is most similar to a linear 20S RNA (2.5 kb) narnavirus found in Saccharomyces cerevisiae. No virus sequence was found in the P. infestans genome. PiRV-4 could not be cured from the P. infestans host after three generations of growth on antiviral media; it is still not clear if the virus is affecting the pathogenicity of its host. PiRV-4 is designated as a new member of the genus Narnavirus in the family Narnaviridae.