"I am optimistic about the country's economic situation and ... I want to reassure our people that foreign currencies will not keep going up," Rouhani said in a live interview with state television.
Iran's rial hit a record low against the US dollar last Monday in a sign of concern about the country's ability to attract foreign money after US President-elect Donald Trump takes office on Jan. 20.
If the rial's weakness continues, it could become a political issue ahead of Iran's presidential election in May by threatening some of Rouhani's economic achievements. Rouhani, who took office in 2013, is expected to seek re-election but has not announced it officially
Economists have said Trump's election in November was a major factor in the rial's weakness. He has said he will scrap the deal between Iran and world powers that imposed curbs on Tehran's nuclear projects and lifted sanctions on the Iranian economy last January.
"The dollar has gained against all currencies in the past few months and it is natural if it also gains in Iran. But the rate of exchange of foreign currencies will certainly not remain at current levels," Rouhani said, without giving details of how the government plans to defend the rial.
"The stability of (currency) markets is important for the government. The economy has to be predictable so that exporters and importers can act with trust," he said.
Before Rouhani's remarks, the rial strengthened to about 39,000 to the dollar on the free market on Sunday, after sliding to an all-time low of 41,500 last Monday.
The rial's gains came after central bank Governor Valiollah Seif told state television on Saturday that he saw 36,000 rials per dollar as a "realistic" rate.