The mumps virus (MuV) is the virus that causes mumps. MuV contains a single-stranded, negative-sense genome made of ribonucleic acid (RNA). Its genome is about 15,000 nucleotides in length and contains seven genes that encode nine proteins. The genome is encased by a capsid that is in turn surrounded by a viral envelope. MuV particles, called virions, are pleomorphic in shape and vary in size from 100 to 600 nanometers in diameter. One serotype and twelve genotypes that vary in their geographic distribution are recognized. Humans are the only natural host of the mumps virus.

MuV replicates first by binding to the surface of cells, whereby its envelope merges with the host cell membrane to release the capsid inside of the cell. Once inside, the viral RNA-dependent RNA polymerase transcribes messenger RNA (mRNA) from the genome and later replicates the genome. After translation of viral proteins, virions are formed adjacent to the cell membrane, where they then leave the cell by budding from its surface, using the cell membrane as the envelope.

The mumps virus was first identified as the cause of mumps in 1934 and was first isolated in 1945. Within a few years after isolation, vaccines protecting against MuV infection had been developed. MuV was first recognized as a species in 1971, and it has been given the scientific name Mumps orthorubulavirus. It is assigned to the genus Orthorubulavirus in the subfamily Rubulavirinae, family Paramyxoviridae.