JC virus or JC polyomavirus is a type of human polyomavirus, genetically similar to BK virus and SV40. It was discovered in 1971 and named after the initials of the person in whom it was found, a patient with progressive multifocal leukoencephalopathy.

The virus is very common in the general population, infecting 70 to 90 percent of humans. Initial infection, which may be through the respiratory tract or orally, produces no symptoms; the virus usually remains latent in the respiratory system and/or kidneys, and can also infect the central nervous system and gastrointestinal tract. Normally, reactivation of the virus will occur only if the patient is immunodeficient, as in AIDS, or immunosuppressed, as in organ transplant patients. If the virus is reactivated, it can attack the kidneys, causing cystitis and ureteral stenosis, or the brain, causing the fatal PML.

Studies since 2000 have found JC virus in malignant colon tumors and suggested that the virus may be linked to colon cancer.