Rosatom spokesman Sergei Novikov announced scientists would load fuel to start operations at the Bushehr plant next weekend, Russia's ITAR-Tass news agency reports.
"The fuel will be loaded on Aug. 21," he was quoted as saying.
Moscow has worked with Tehran on construction of the Bushehr power plant since the 1990s. Rosatom will operate the plant, supply the fuel and remove any nuclear waste.
Officials said that action means Iran will have little chance to exploit nuclear technology at Bushehr to develop a nuclear weapon.
The U.N. Security Council in June imposed new sanctions on Iran as punishment for its controversial nuclear program. Moscow backed those sanctions.
Washington and the European Union followed suit with unilateral sanctions in July that target Iran's energy sector.
Western allies worry Iran is developing the technology needed to develop a nuclear weapon. Tehran insists it nuclear program is for civilian energy.
Iran sits on some of the largest oil and gas deposits in the world. Crumbling infrastructure, however, forces Tehran to rely on imports to meet some of its energy needs.
A date for full operations at Bushehr isn't known.