Russia banned oil sales to countries that complied with the price cap.
The price cap came into effect on December 5th.
The cap prevents countries from paying more than $60 per barrel.
Russia's oil and oil products won't be sold to anyone who imposes a price cap.
The ban will be in effect for five months.
Russia could give special permission to supply to countries that are under the ban according to the decree.
The idea of a price cap was put forward by the G7 in order to stop Moscow from using oil revenue to fund the war in Ukraine.
Russian revenue remained high even after the invasion because of a price spike and demand from India and China.
An EU-wide ban on Russian crude oil imported by sea is already in place.
Russian oil revenue has been reduced by the price cap. Any Russian crude sold for more than $60 is stopped from being shipped.
Shipping and insurance companies are based in the G7.
Ukrainian President Volodymyr Zelensky said the price cap was not serious enough to hurt the Russian economy.
Russia's budget deficit could be larger than planned because of the oil price cap, according to the Finance Minister.
The price of oil has fallen from the peaks of above $120 seen in March and June.