The drought which has affected much of Europe this summer forced the authorities to close the station down at the end of August because the level of the Danube dropped to a point where the river water could no longer be used to cool the reactor.
"The waters of the Danube.. have reached the level required for the long term operation of the station," a statement from the ministry, responsible for industry and energy, said. The station will come back into service Wednesday.
The reactor, which provides 10 percent of the country's energy, was closed down on August 23 when the Danube's flow at the point where it enters Romania fell to less than 1,600 cubic metres a second.
On Tuesday the rate of flow had reached 1,930 cubic metres a second.
The temporary closure of the reactor, the only one in eastern Europe to use western technology, cost the operating company Nuclearelectrica 360,000 dollars (euros)a day.
The Cernavoda station is planned eventually to have five reactors but at present only one, with a capacity of 705 megawatts, is in service.
The drought in Romania was the worst for a century and led to a reduction of output from hydroelectric stations, forcing the govermment to rely more heavily on stations using coal and oil.