Japanese firm Nippon Oil is to start paying for Iranian oil in yen, rather than in US dollars.
Exporting oil provides a vital source of revenue for Iran
The first payments to be made in the new currency for crude oil contracts will take place in October.
Iran has been increasingly selling oil in currencies other than the US dollar, which has fallen in value.
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.
The move is not intended to change the original value of the oil contracts being traded.