A Russian company leading construction of the plant near Iran's southern port of Bushehr, earlier this year delayed its launch, which had been set for September, saying Teheran was behind schedule on payments.
But Atomstroiexport said in April that it had agreed on a financing plan with Teheran, setting the stage for the announcement.
"Bushehr nuclear power plant will be launched in October, according to schedule," Iranian television quoted Energy Minister Parviz Fattah as saying. "Power substations and lines for supplying electricity, which would be produced by the plant, are ready to use."
The international community fears Iran could be seeking to develop nuclear weapons. Teheran insists its nuclear development is meant only for electricity production.
Iranian officials had earlier denied any payment delays, and accused Russia of caving in to Western pressure.
Moscow has cultivated close ties with Iran, but has supported limited UN sanctions against Teheran, while warding off US efforts to level harsher punishments.
But Iran has irritated Moscow by turning a cold shoulder to its efforts to resolve the persistent confrontation over its nuclear program, including an offer to enrich uranium for Iranian plants in Russia, which could provide Iran with nuclear fuel while easing concerns it might develop weapons.
Construction of the Bushehr plant began in 1974 with help from then-West Germany. Work was then interrupted during the 1979 Islamic revolution that toppled the pro-Western Shah Mohammad Reza Pahlavi and brought hard-line clerics to power. Iraq also bombed the plant during its 1980-88 war with Iran.
When Iran tried to resume the project after the war, the Germans refused to help. Iran turned to Russia, signing a US $1 billion contract to build the 1,000-megawatt Bushehr plant in 1995. It was first scheduled to open in 1999, but has suffered many delays.