Poliovirus, the causative agent of poliomyelitis, is a human enterovirus and member of the family of Picornaviridae. Poliovirus is composed of an RNA genome and a protein capsid. The genome is a single-stranded positive-sense RNA genome that is about 7500 nucleotides long. The viral particle is about 30 nanometres in diameter with icosahedral symmetry. Because of its short genome and its simple composition—only RNA and a non-enveloped icosahedral protein coat that encapsulates it—poliovirus is widely regarded as the simplest significant virus.

Poliovirus was first isolated in 1909 by Karl Landsteiner and Erwin Popper. In 1981, the poliovirus genome was published by two different teams of researchers— by Vincent Racaniello and David Baltimore at MIT and by Naomi Kitamura and others at the State University of New York, Stony Brook. Poliovirus is one of the most well-characterized viruses, and has become a useful model system for understanding the biology of RNA viruses.