- An oncovirus is a virus that can cause cancer
- This term originated from studies of acutely transforming retroviruses in the 1950–60s, when the term "oncornaviruses" was used to denote their RNA virus origin
- With the letters "RNA" removed, 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 longstanding co-evolution between the virus and its host
- Oncoviruses have been important not only in epidemiology, but also in investigations of cell cycle control mechanisms such as the retinoblastoma protein