This review explores the presence and functions of polyglutamine (polyQ) in viral proteins. In mammals, mutations in polyQ segments (and CAG repeats at the nucleotide level) have been linked to neural disorders and ataxias. PolyQ regions in normal human proteins have documented functional roles, in transcription factors and, more recently, in regulating autophagy. Despite the high frequency of polyQ repeats in eukaryotic genomes, little attention has been given to the presence or possible role of polyQ sequences in virus genomes. A survey described here revealed that polyQ repeats occur rarely in RNA viruses, suggesting that they have detrimental effects on virus replication at the nucleotide or protein level. However, there have been sporadic reports of polyQ segments in potyviruses and in reptilian nidoviruses (among the largest RNA viruses known). Conserved polyQ segments are found in the regulatory control proteins of many DNA viruses. Variable length polyQ tracts are found in proteins that contribute to transmissibility (cowpox A-type inclusion protein (ATI)) and control of latency (herpes viruses). New longer-read sequencing methods, using original biological samples, should reveal more details on the presence and functional role of polyQ in viruses, as well as the nucleotide regions that encode them. Given the known toxic effects of polyQ repeats, the role of these segments in neurovirulent and tumorigenic viruses should be further explored.