The o'nyong'nyong virus or o'nyong-nyong virus is a virus first isolated by the Uganda Virus Research Institute in Entebbe, Uganda in 1959. It is a togavirus (family Togaviridae), genus Alphavirus and is closely related to the chikungunya and Igbo Ora viruses. The name comes from the Nilotic language of Uganda and Sudan and means “weakening of the joints". The virus can infect humans and may cause disease.

O'nyong'nyong virus is transmitted by bites from an infected mosquito. It is the only virus whose primary vectors are anopheline mosquitoes (Anopheles funestus and Anopheles gambiae).

Common symptoms of infection with the virus are polyarthritis, rash and fever. Other symptoms include eye pain, chest pain, lymphadenitis and lethargy. No fatalities due to infection are known.

There have been two epidemics of o’nyong’nyong fever. The first occurred from 1959 to 1962, spreading from Uganda to Kenya, Tanzania, Zaire (Democratic Republic of the Congo), Malawi and Mozambique, and affecting over two million people. This was one of the largest arbovirus epidemics recorded. The first virus isolates were obtained during this outbreak from mosquitoes and human blood samples collected from Gulu in northern Uganda in 1959.