The former Massachusetts governor, who is narrowly trailing in polls less than two months before the election, has repeatedly criticized Obama over ties with the rising Asian power on issues from trade to human rights to defense.
Addressing a rally in the critical state of Virginia, Romney pledged to open new markets for US goods and said: "I also want to make sure that if a nation cheats like China has cheated, we call them on the carpet and don't let it continue."
Romney said that China's alleged undervaluing of its currency makes its products artificially cheap and "that then drives American manufacturers and American producers out of business and kills jobs."
"The president's had the chance year after year to label China a currency manipulator, but he hasn't done so. And I will label China the currency manipulator they are on the first day," Romney said.
The Treasury Department, in semiannual reports, has resisted pressure to declare China a currency manipulator, a designation that would pave the way for sanctions and, according to some analysts, could set off a trade war between the world's two largest economies.
Treasury Secretary Timothy Geithner has preferred quiet diplomacy to nudge China to let its currency appreciate. The Obama administration, despite frequent friction with China, has hoped to preserve cooperation with Beijing on issues such as the global economy, North Korea, Iran and climate change.
Asked about Romney's charges, Ambassador to China Gary Locke said he would "not engage in political debates" but said that the US position "is very clear and has been very consistent" in urging China to reform its exchange rate.
"We know that the currency is still undervalued," Locke said at the Carnegie Endowment for International Peace during a visit to Washington.
"The Chinese have made substantial progress and when you factor in the rate of inflation, it has appreciated significantly. But, again, still more needs to be done," said Locke, a former governor from Obama's Democratic Party.
The yuan, also known as the renminbi, has appreciated more than five percent against the dollar in the past two years as China contends with inflationary pressure. The yuan has risen some 30 percent since 2005 when China loosened its grip on it.
Critics in the United States and other developed economies accuse China of keeping its currency deliberately low to flood the world with exports of inexpensive goods, devastating the manufacturing industry elsewhere.
Romney's campaign has pointed out that Obama, as a candidate in 2007, vowed to take China "to the mat" over its currency.
The charges on China come as foreign policy increasingly enters the presidential race, with Romney criticizing Obama's stance on violent protests in the Middle East triggered by an amateurish anti-Islam film on the Internet.
Romney set off a storm of criticism even by some Republicans after he accused the Obama administration of "sympathizing" with protesters through a statement by the US embassy in Cairo that condemned efforts to offend Muslims.
Four Americans including the US ambassador to Libya, Chris Stevens, were killed in a separate attack on the US consulate in Benghazi.