Iran, the fourth-biggest oil exporter, has made the shift in the light of political differences with the US over its nuclear programme.
While Iran says the project is for civilian purposes only, the US argues it is to develop nuclear weapons.
Last year, Iran inserted a clause into oil contracts enabling it to require payment in currency other than the US dollar.
Iran and other countries that rely heavily on oil exports have been hard hit by the decline in the dollar's value.
