An oncovirus is a virus that can cause cancer. This term originated from studies of acutely-transforming retroviruses in the 1950–60s, often called oncornaviruses to denote their RNA virus origin. It now refers to any virus with a DNA or RNA genome causing cancer and is synonymous with "tumor virus" or "cancer virus". The vast majority of human and animal viruses do not cause cancer, probably because of long-standing coevolution between the virus and its host.

Worldwide, the WHO International Agency for Research on Cancer estimated that in 2002 17.8% of human cancers were caused by infection, with 11.9% being caused by one of seven different viruses. The importance of this is that these cancers might be easily prevented through vaccination (e.g., papillomavirus vaccines), diagnosed with simple blood tests, and treated with less-toxic antiviral compounds.